Natural sea-salt onsen and an ice-cold groundwater cold plunge bath
Yamanashi Fujikawaguchiko Town
Izumi no Yu, located on the shore of Lake Saiko in Fujikawaguchiko, is a day-use hot spring facility where you can enjoy a rare onsen containing natural sea salt. Relax in the large communal baths and open-air baths while taking in the gifts of nature. The facility features a dry sauna at around 90°C. A standout feature is the cold plunge bath, which uses high-quality well water flowing from beneath Lake Saiko; it is run without a chiller and offers superb, piercingly cold water at around 13°C.
TypeNatural Hot Spring
An onsen of the calcium–sodium sulfate spring type that contains natural sea salt. Its mineral composition is similar to thalassotherapy (marine therapy), providing restorative benefits for body and mind.
Sauna TypeDry Sauna
Stove Typeelectric
Capacity10people
A dry sauna around 90°C where you can work up a good sweat. Capacity limits may be in place due to COVID-19 or other public health measures.
Water QualityNatural Water (Groundwater, Spring, etc.)
Capacity2people
This cold plunge bath is filled with mineral-rich groundwater drawn from beneath Lake Saiko, which supports the Kunimasu, a species found only in Lake Saiko. Even without a chiller, the water is bracingly cold.
Closed Days: Tuesday, Thursday, Wednesday
987 Saiko, Fujikawaguchiko-machi, Minamitsuru-gun, Yamanashi 401-0332, Japan
Gender Access
Gender Separated
Curated by an expert who visits onsen and sauna facilities every day and has explored 300+ locations — only facilities confirmed to be truly exceptional are featured.
Learn more
Natural sea-salt onsen and an ice-cold groundwater cold plunge bath
Yamanashi Fujikawaguchiko Town
Izumi no Yu, located on the shore of Lake Saiko in Fujikawaguchiko, is a day-use hot spring facility where you can enjoy a rare onsen containing natural sea salt. Relax in the large communal baths and open-air baths while taking in the gifts of nature. The facility features a dry sauna at around 90°C. A standout feature is the cold plunge bath, which uses high-quality well water flowing from beneath Lake Saiko; it is run without a chiller and offers superb, piercingly cold water at around 13°C.
TypeNatural Hot Spring
An onsen of the calcium–sodium sulfate spring type that contains natural sea salt. Its mineral composition is similar to thalassotherapy (marine therapy), providing restorative benefits for body and mind.
Sauna TypeDry Sauna
Stove Typeelectric
Capacity10people
A dry sauna around 90°C where you can work up a good sweat. Capacity limits may be in place due to COVID-19 or other public health measures.
Water QualityNatural Water (Groundwater, Spring, etc.)
Capacity2people
This cold plunge bath is filled with mineral-rich groundwater drawn from beneath Lake Saiko, which supports the Kunimasu, a species found only in Lake Saiko. Even without a chiller, the water is bracingly cold.
Closed Days: Tuesday, Thursday, Wednesday
987 Saiko, Fujikawaguchiko-machi, Minamitsuru-gun, Yamanashi 401-0332, Japan
Gender Access
Gender Separated
Curated by an expert who visits onsen and sauna facilities every day and has explored 300+ locations — only facilities confirmed to be truly exceptional are featured.
Learn more