Kawayu Onsen Sennin-buro

What to expect

Kawayu Onsen is a one-street hot spring village on the Oto River, a tributary of the Kumano River, in the Hongu area of Tanabe city in southern Wakayama. Its trick is that the hot water comes up through the riverbed itself. Spring water at about 163°F73°C seeps through the gravel, and wherever you dig a hole on the bank, it fills with hot water. Inns line the north bank; the south side is steep forest.

Every winter the local tourism association goes one step further. It walls off part of the river to make Sennin-buro, a single open-air pool big enough for crowds. River water is let in to bring the hot spring down to about 104°F40°C. The Tanabe City Kumano Tourism Bureau gives its size as about 40 by 15 m and about 60 cm deep, though the Hongu association points out that it is rebuilt each year by hand and its size changes from season to season. “Sennin” is a play on words: it can mean a thousand people, or a mountain hermit.

The pool has no roof and only reed screens around it, so you soak under open sky by day and stars by night. The temperature varies from spot to spot. Move around until you find a comfortable patch, and watch out for hotter water near the spring vents.

Facilities are basic: makeshift changing huts on the riverbank (the women’s is enclosed; the men’s is open on the mountain side, according to the tourism bureau), a public toilet across the Kawayu Kaitaku Bridge, and a free car park.

The water

The Hongu association lists Kawayu’s water as a sodium bicarbonate and chloride spring at 163°F73°C at source. The Sennin-buro is not heated, treated or circulated in any way it describes: it is spring water and river water mixing in the open. That also means its temperature depends on how much river water is flowing, and after rain it can drop too far to bathe in.

Rules and etiquette

The tourism association’s rules for the bath, in short:

• Swimsuits are compulsory. This is a river and a mixed-gender bath. The association says nude bathing could breach Article 174 of the Penal Code. A towel wrap over a swimsuit is fine.
• No soap or shampoo in the river.
• No eating or drinking in the bath, and absolutely no alcohol.
• No pets past the bridge on the Sennin-buro side.
• No goggles, no smoking, and no photos of other bathers without asking.
• Take your rubbish home.
• Do not bathe if you feel unwell or have been drinking, and ease into the water slowly.
• Keep to the hours: last entry 21:30, everyone out by 22:00.

The association accepts no responsibility for theft or accidents, so leave valuables at your inn or in the car.

The association publishes no tattoo policy. Call 0735-42-0735 if you want to be sure before you travel.

Getting there

Kawayu Onsen is a short drive from Kumano Hongu Taisha, the main shrine on the Kumano Kodo pilgrimage routes. Buses run here every day from Kii-Tanabe and Shirahama, from Shingu, and from the Hongu Taisha-mae stop; the Tanabe tourism bureau posts current timetables. Get off at Kawayu Onsen.

By car, park in the free lot about 200 m downstream of the bath (the Hongu association counts about 36 spaces; the tourism bureau says about 50). Walk upstream along the riverbank to the pool.

When to go

Sennin-buro is a winter event, open from December to the end of February. The 2025-26 season ran from December 1, 2025 to February 28, 2026, the usual pattern. As of October 22, 2026 no dates had been posted for 2026-27. The association warns that the opening may slip if the weather does not cooperate, and it shows the current status (“温泉営業状況”) on the home page of hongu.jp.

Within the season there are no fixed closing days, but heavy rain can close the bath for a day or more, and it occasionally closes for cleaning. On Saturday evenings during the season, roughly from 20:00 to 22:00, lanterns are set out along the riverbank (cancelled in bad weather). Weekday evenings are quieter.

Outside winter, Kawayu is still worth the trip. You can dig your own bath in the gravel bar along the river, then cool off in the river in summer. Guests of local inns can usually borrow a shovel; anyone else should bring one. Some inns also make their own riverside pools.

The public bathhouse

The Kawayu Onsen Public Bathhouse (Kawayu Onsen Koshu Yokujo) sits on the riverside in the middle of the inn street and pipes in water straight from a riverbed source. It is a simple local bath, split by gender with normal nude bathing rules. Hours are 6:30 to 20:00 (last entry 19:30), closed Tuesdays. Entry is ¥300 for adults (junior high and over), ¥200 for children and ¥80 for infants; soap and shampoo are ¥50 each and a towel ¥200 (prices as listed in 2023). It is a good place to warm up properly and rinse off after Sennin-buro.

Where to stay nearby

Kawayu has a row of ryokan, minshuku and hotels on the north bank, most with river views, and some open their baths to day visitors. Most inns keep swimsuits and sandals for guests, sometimes for a small fee, which is useful if you arrive without them. For camping, the Hongu association points to Tanabe Kawayu Campground at Kawayu Onsen. The neighbouring hot spring villages of Yunomine Onsen, home to the tiny Tsuboyu bath, and Wataze Onsen are part of the same Hongu hot spring area and make easy additions.

Safety notes

• Hot spots: the vents in the riverbed can be much hotter than the rest of the pool. Test the water with your hand before sitting down, especially with children.
• Sharp rocks: the bottom is river stones. Wear sandals or water shoes.
• Cold and heat: winter nights here are cold, and moving between hot water and cold air is hard on the body. Keep soaks short, get out slowly and dress quickly.
• River conditions: do not bathe if the river is high or murky after rain, even if the pool looks open. Staff close it for good reason.
• Alcohol: banned in the bath. Do not soak after drinking.
• Valuables: nobody watches the changing huts. Bring only what you need.