Masutomi Onsen: Yamanashi Radon Hot Spring

Masutomi Onsen is a remote mountain hot spring with a history of over 1,200 years. Its tepid waters, containing some of the highest radium concentrations in Japan, are known as therapeutic waters that heal you from the core when you soak slowly and at length. In a quiet setting embraced by rich nature, it is a hidden gem where you can refresh both body and mind, and where a genuine culture of toji (extended hot-spring therapy) still lives on.

Highlights

About Masutomi Onsen

Masutomi Onsen is a hot spring area nestled along a deep mountain gorge in Hokuto City, Yamanashi. Located within Chichibu-Tama-Kai National Park, it retains legends connected to Takeda Shingen gold-mining ventures. Designated a National Health Resort Hot Spring in 1965, it has been loved by literary figures including the haiku poet Takahama Kyoshi. Away from the bustle, it is a precious remote spring where genuine hot-spring therapy can be enjoyed.

High-Radium Tepid Waters

Masutomi Onsen greatest feature is being a radioactive spring with some of the highest concentrations in Japan. The water is a sodium chloride and bicarbonate spring containing carbon dioxide. The source temperature is a low, tepid 30 degrees Celsius, and the Masutomi way is to soak slowly for 15 to 30 minutes. Long bathing lets the body gradually take in the effects, warming you from within. The lingering warmth after bathing is a comfort unique to tepid springs.

Therapy and Nature

Masutomi Onsen preserves the atmosphere of an old-fashioned therapy resort. A handful of inns operate along the gorge, and you can experience genuine multi-day therapy stays that restore the body through hot springs and rest. The surroundings, ringed by mountains such as Mizugaki-yama, are rich in nature and especially beautiful in the fresh-green and autumn-foliage seasons. Time spent listening to the murmur of the mountain stream offers supreme healing that makes you forget everyday fatigue.

Seasonal Highlights

Early summer fresh greenery (May to June) and autumn foliage (October to November) are especially beautiful and ideal for restful stays. Summer is cool at this high elevation, perfect for therapy that doubles as an escape from the heat. In winter you can savor a quiet moment soaking in the tepid waters amid snowy scenery. Many visitors combine a trip with the climbing seasons for Mizugaki-yama and Kinpu-san, enjoying nature different faces through the seasons.

Access and Basic Information

For access, take a Yamanashi Kotsu bus about 60 minutes from Nirasaki Station on the JR Chuo Main Line. As a remote spring reached along mountain roads, arriving by car is also convenient, taking about 40 minutes via prefectural roads from the Sudama Interchange on the Chuo Expressway. Several hot-spring inns line the gorge, and staying overnight for therapy is the norm. Day-use bathing availability and operating status can change, so check with each inn in advance to be safe.

Practical Travel Notes

The best seasons are the fresh-green and autumn-foliage periods. Since tepid springs assume long soaks, an unhurried stay is recommended. Rather than rushing through in half a day, staying overnight to relax into the therapy mood lets you enjoy the true appeal. Because the drive follows mountain roads, those unused to such driving may prefer the bus. Nearby are Mizugaki-yama, one of Japan 100 Famous Mountains, Lake Mizugaki (Shiokawa Dam), and Mizugaki-yama Nature Park, which pair well with nature walks. Budget-wise, day-use bathing is a few hundred to around a thousand yen, while overnight rates vary by plan.

A Word of Advice

Masutomi Onsen is a fine tepid spring that shows its true value only with long, patient soaks. The trick is not to cut things short but to warm the body over time. Bring a change of clothes and something to layer on, and take care not to catch a chill afterward. In the quiet gorge, spend a luxurious stretch of time gently unwinding body and mind.

Visitor Checklist

• Estimated time: The tepid baths are meant for long soaking, so an overnight stay best captures the cure.
• Bathing tip: The spring is a mild 30 degrees; the Masutomi way is to soak slowly for 15 to 30 minutes.
• Travel note: The approach is a mountain drive, so those unsure of it may prefer the bus.
• Best season: Early-summer greenery and autumn colour are especially fine and ideal for a rest cure.
• Nearby stops: Combine with Mount Mizugaki, Lake Mizugaki, and the Mizugaki nature park.

Practical Information for Your Visit

• Location: Hishi and Obi in Sutama-cho, Hokuto City, Yamanashi Prefecture, and the surrounding area (the inns are scattered along the valley)
• Day-use bathing: The day-use facility "Masutomi no Yu" has been closed since April 2023 (for rebuilding). For day visits, use the drop-in baths at the ryokan
• Day-use bathing at Furokaku: 12:30-14:00, up to 1 hour per visit, 1,000 yen per person. Not available on Tuesdays or on the inn's own closed days
• Spring quality: A radioactive spring containing radium. The source temperature is around 30 degrees, making it a lukewarm bath
• How long to soak: A long soak of 15 to 30 minutes is the norm. Allowing about an hour including changing gives you room to spare
• Access (train and bus): About 60 minutes by Yamako Town Coach bus from Nirasaki Station on the JR Chuo Main Line
• Access (car): About 30 to 40 minutes from the Sutama IC on the Chuo Expressway via prefectural roads. The road is a continuous mountain road, so allow extra time when driving after dusk or in winter
• How long to stay: Half a day for a day trip. If you want to enjoy long soaks in the lukewarm water as they are meant to be enjoyed, an overnight stay is recommended
• In case of rain: Since bathing is the main purpose, rain is no obstacle. However, driving the mountain roads and hiking or walking the valley nearby do depend on the weather
• What to wear and bring: You will sweat during a long soak, so bring something to drink. You chill easily after bathing, so it is reassuring to have something to put on
• Budget guide: Day-use bathing is around 1,000 yen. Accommodation ranges from self-catering hot spring cure plans to full ryokan plans
• Nearby: Mt. Mizugaki, one of Japan's 100 Famous Mountains, Mizugakiyama Nature Park, and Lake Mizugaki (Shiokawa Dam). Many people combine a visit with a climb of Mt. Mizugaki or Mt. Kinpu
• Strollers and wheelchairs: The inns stand on sloping ground in the mountains, and facilities differ from inn to inn. If you have particular needs, check with each inn in advance

📚 Sources

This article draws on tourism guidance for Masutomi Onsen in Hokuto City and information published by its inns, materials on its status as a National Health Resort within Chichibu-Tama-Kai National Park, and climbing information for nearby Mount Mizugaki. Whether day-use bathing is offered, its fees, lodging plans, and forest-road and transport conditions vary by season and facility. Since fees and times may change, please confirm each inn's official information before you visit.

The reason for the closure of "Masutomi no Yu", the policy on rebuilding and the outlook for reopening come from information on the official Hokuto City website, and access by car and bus comes from the tourism pages of the official Hokuto City website. The hours and fees for day-use bathing are based on information on the official Furokaku website. The designation as a National Health Resort Hot Spring and the number of lodgings and their capacity refer to information published by the Japan Spa Association. Opening hours, prices and availability are subject to change, so please check the official information before you set out.