Nagayu Onsen is a hot spring area along the Serikawa River in Naoi, Taketa City, Oita Prefecture, and its biggest feature is that it is known as one of Japan’s highest-concentration natural carbonated springs. The water contains a large amount of free carbon dioxide, and it is said to be a spring that is good both to drink and to bathe in. When you soak in it, tiny bubbles cling to your skin. Attractions tied to the carbonated spring include Ramune Onsenkan, designed by architect Fujimori Terunobu, and Gani-yu, an open-air bath set in the middle of the Serikawa River.

For travelers, the key point is that this is not simply a famous hot spring town, but a place to experience an unusual carbonated spring firsthand. Because carbon dioxide escapes more easily at high temperatures, carbonated springs are often served lukewarm, and Nagayu is centered on baths where you can soak slowly and enjoy the bubbles. This article organizes Nagayu Onsen’s features, the appeal of its carbonated spring, Ramune Onsenkan and Gani-yu, its ties with Germany, and access from Beppu, Yufuin, and Aso, based on materials from Taketa City and the Ministry of the Environment. For the scientific overview of spring types, see Onsen Spring Types for Beginners.

This article is for general information only and does not guarantee any specific medical or health benefits. The use of carbonated springs or drinking spring water, as well as facility fees and opening hours, may change, so please check official information before visiting. If you have a chronic condition or are feeling unwell, do not overexert yourself and follow the advice of your doctor and the facility staff.

Features of Nagayu Onsen: A carbonated spring village along the Serikawa River

Nagayu Onsen is located in Naoi, Taketa City, near the southwestern foot of the Kuju Mountains in central Oita Prefecture. Inns and public baths are scattered along both banks of the Serikawa River. Unlike large hot spring towns such as Beppu or Yufuin, it has the calm atmosphere of a riverside village, with springs notable for their strong identity as carbonated water.

What made Nagayu famous nationwide is its spring quality. It is generally described as a magnesium-sodium bicarbonate spring and is renowned as a high-concentration carbonated spring rich in free carbon dioxide. Japan is a volcanic country where spring temperatures tend to be high, and carbon dioxide dissolves less easily in hot water, so naturally occurring high-concentration carbonated springs are limited. In that context, Nagayu has long been introduced as “Japan’s best carbonated spring” because it contains plenty of carbon dioxide even at relatively high temperatures. The mechanism and standards of carbonated springs are explained in detail in Carbonated Onsen (Carbon Dioxide Springs): Features and Famous Baths.

What does “good to drink and good to bathe in” mean?

When people talk about Nagayu Onsen, they often use the phrase “good to drink and good to bathe in.” This long-familiar nickname reflects its history of being used both for bathing and for drinking spring water. In this article, the phrase is not treated as a guarantee of any specific effect.

The most visible sign of a carbonated spring is that carbon dioxide dissolved in the water attaches to the body and appears as tiny bubbles. It is similar to the bubbles that cling to the inside of a glass of carbonated drink, which is why it is sometimes described as “ramune water.” The amount of bubbling varies depending on concentration, temperature, and freshness, and it is easier to observe when you soak quietly.

Nagayu also has a long-established drinking-spring culture. However, drinking spring water should only be done where it is explicitly permitted and designated as drinkable, and each facility has its own guidelines for amount and frequency. Please do not casually drink directly from a source while sightseeing; follow local instructions. The basics of drinking spring water are explained in Drinking Onsen Water Culture. Because carbon dioxide escapes more easily at high temperatures, lukewarm baths are common in Nagayu, which may feel too mild for people who prefer hot water, but it suits those who want to soak slowly in warm water.

Quick look at the main sights

The main sights of Nagayu Onsen, organized by location, features, and approximate time needed, are as follows. These times are only rough estimates and may vary depending on crowds and opening hours.

Fees, opening hours, and closing days for each facility may change, so checking the official information before visiting is the safest choice.

Ramune Onsenkan: A landmark building designed by Fujimori Terunobu

The facility that symbolizes Nagayu Onsen is Ramune Onsenkan. It is known as a building designed by architect Fujimori Terunobu, and its striking black-and-white exterior and pointed roof catch the eye. The architecture itself is an attraction, making it one of the destinations worth visiting in Nagayu.

Fujimori Terunobu is an architect known for bold use of natural materials and buildings that combine humor with a sense of local identity. At Ramune Onsenkan, he created a memorable form that blends into the surrounding satoyama while still standing out, with black burned-cedar walls, white plaster stripes, and plantings on the roof. The way hot spring architecture designs not only the quality of the water but also the spatial experience is also discussed in Onsen Ryokan Architecture.

Inside, there is a lukewarm outdoor bath where the carbon dioxide can be felt strongly, as well as an indoor bath with a higher temperature, letting visitors experience the carbonated spring differently depending on the heat. As a day-use facility, it can be used without staying overnight, making it ideal for a first taste of Nagayu’s carbonated spring. Opening hours, closing days, and bathing fees may change, so please check the official information before visiting.

Gani-yu: An open-air bath in the middle of the Serikawa River

Another local specialty is Gani-yu, an open-air bath in the middle of the Serikawa River. Rather than being on the riverbank, the tub is set directly in the river, creating an open structure with nothing blocking the view. It is often photographed as a symbol of Nagayu’s carbonated spring scenery.

However, because Gani-yu is an open-air bath with no roof or enclosure, there are many points to confirm before using it, such as the time of day, whether swimwear is allowed, crowding, and safety during high water. Usage conditions, fees, and available times may change, and this article does not make any definitive claims. If you plan to go in, be sure to check official information or on-site guidance for whether it is available and what precautions apply. The experience of soaking in an open bath in nature is one of Nagayu’s great appeals, but precisely because it is open, consideration for others and safety checks are essential.

Exchange with German carbonated spring resorts

Nagayu Onsen is also known for its exchange with carbonated spring resorts in Germany. Europe has relatively many natural carbonated springs, and in Germany a spa culture using carbonated springs has developed. In Japan, a volcanic country with many hot springs and rare high-concentration carbonated springs, the geological background is different from Germany, where carbonated springs are widely distributed.

In Nagayu, people have talked about staying and wellness initiatives that make use of hot springs, drawing on this idea of carbonated spring therapy. This is strictly in the context of regional development and cultural exchange, and this article does not claim any medical effect. It is worth remembering that a common spring type can create cross-border ties, and that is part of what makes Nagayu distinctive.

Access

Nagayu Onsen is far from train stations, so travel by car is the norm. The times below are rough estimates and will vary depending on schedules and road conditions.

From Oita and Beppu, the usual route is by car through the southern foot of the Kuju Mountains, and it is easy to combine the trip with sightseeing in Beppu or Yufuin Onsen. You can extend your trip from Beppu Onsen, famous for its hell tours, and experience a different kind of spring with a carbonated profile. It also connects well with the Taketa, Kuju Plateau, and Aso areas, making it easy to build a tour that combines the natural beauty of the Kuju Mountains and Aso with inland Oita sightseeing.

If using public transportation, you would typically start from the nearest train station or bus route and then take a bus or taxi, and service frequency may be limited. Checking timetables in advance will help you avoid rushing on site. Since Nagayu is a place to savor carbonated springs slowly, it is better not to cram in too much sightseeing and instead secure enough time to soak in the lukewarm water.

FAQ

Why is Nagayu Onsen called “Japan’s best carbonated spring”?

Naturally occurring high-concentration carbonated springs are limited in Japan, a volcanic country. This is because carbon dioxide does not dissolve easily in hot water. Nagayu is known for having a large amount of free carbon dioxide even at relatively high temperatures, which is why it has been described that way. However, concentrations differ from one source to another even within the same hot spring area, so the safest way to confirm is by checking the listed spring quality.

Does “good to drink and good to bathe in” guarantee any benefits?

No. It is a long-familiar nickname in Nagayu that reflects the history of using the spring both for bathing and drinking. This article does not claim any specific benefits. Drinking spring water should only be done where it is explicitly allowed, and you should follow the local guidance on amount and frequency.

Who designed Ramune Onsenkan?

It was designed by architect Fujimori Terunobu. The building is characterized by its black-and-white exterior and pointed roof, and it is a day-use facility with a lukewarm outdoor carbonated bath and a hotter indoor bath. Opening hours and bathing fees may change, so please check the official information before visiting.

Can anyone enter Gani-yu freely?

Gani-yu is an open-air bath in the middle of the Serikawa River, and there are many points to confirm, such as available hours, whether swimwear is allowed, and safety during high water. Usage conditions may change, so this article does not make any definitive claims. If you plan to go in, be sure to check official information or on-site guidance for availability and precautions.

Why is carbonated spring water lukewarm?

Because carbon dioxide escapes from water more easily as the temperature rises. At high temperatures, the carbonation tends to disappear, so lukewarm baths are often prepared to make the bubbles and carbonated sensation easier to enjoy. It may feel too mild for people who like hot water, but it suits those who want to soak slowly in warm water.

Summary

Nagayu Onsen is a hot spring area along the Serikawa River in Naoi, Taketa City, Oita Prefecture, and is known as one of Japan’s highest-concentration natural carbonated springs. Its great appeal is that you can experience a carbonated spring said to be “good to drink and good to bathe in” at Ramune Onsenkan, designed by Fujimori Terunobu, and at Gani-yu, the open-air bath in the river. Because carbon dioxide escapes more easily at high temperatures, carbonated springs are often served lukewarm, making this a place where you can soak slowly and enjoy the bubbles.

As a traveler, it is best not to make definitive claims about benefits, and instead enjoy the unusual spring quality, the architecture of Ramune Onsenkan, and the sense of openness at Gani-yu together. Since Gani-yu’s usage conditions and the fees and opening hours of each facility may change, checking official information is essential. If you combine it with Beppu, Yufuin, and the Aso area, you can enjoy a wide range of Oita’s hot spring culture in one trip. If you want to compare famous hot springs across Japan, see Top 10 Famous Onsen in Japan as well.

Sources

• Taketa City: Tourism and Events
• Taketa City Tourism and Tourism Association (Nagayu Onsen)
• Ministry of the Environment: Guidelines for Mineral Spring Analysis (Revised 2014)
• Japan Spa Association: Different Types of Onsen Spring Quality