Sainokawara Rotenburo

What to expect

Sainokawara Rotenburo is the big open-air bath of Kusatsu Onsen, in the mountains of Gunma Prefecture. It sits at the top of Sainokawara Park, a valley on the west side of town where hot water comes up from the ground all over the riverbed and runs down as a steaming stream. The tourism association puts the flow from the park at about 1,400 litres a minute. The area is a special zone of Jōshin’etsu Kōgen National Park.

The bath itself is simple. You pay at the front desk, change in the men’s or women’s changing room and walk straight out to a large rock-lined pool surrounded by forest. Together the two sides cover about 500 m², 333.5 m² for men and 166 m² for women, which makes it one of the largest open-air baths in Japan. There is nothing else here: no indoor bath, no washing area, no restaurant and no rest room. People come for the scale, the trees and the snow in winter. In the evening the bath and the hillside of Maruyama are lit up.

The bath opened in 1987 and is run today by the town’s tourism corporation, which also runs Otakinoyu and Gozanoyu. For the rest of Kusatsu, including the free communal baths and the yumomi show, see our Kusatsu Onsen listing.

The water

Sainokawara draws on the Bandai source, which the operator describes as one of Kusatsu’s hottest, coming out of the ground at about 201-203°F94-95°C. By the time it reaches the pools it is cooled enough to bathe in, but the operator does not publish the pool temperature. The water is an acidic chloride-sulfate spring with a pH of 1.7, strongly acidic even by Kusatsu standards. The operator says it is free-flowing straight from the source, with no recirculation. Kusatsu’s acid water can tarnish metal, and the operator warns that metal parts on swimsuits may discolour.

Prices and booking

Prices are from the operator’s site in October 2026.

Ticket	Price
Adult	¥800
Child (age 3 to elementary school)	¥400
Disability certificate holders	half price
Choina three-bath pass (Sainokawara, Otakinoyu, Gozanoyu, one visit each)	¥2,100 adults, ¥1,050 children
Mixed-bathing wrap rental for women (Fridays)	¥400
Face towel / bath towel (to buy)	¥380 / ¥1,700

There is no booking and no time limit. Re-entry on the same ticket is not allowed, and outside food and drink are banned. Valuables lockers are free (you get the coin back), and the larger coin lockers cost ¥100. The front desk takes Visa, Mastercard, JCB, Amex and AEON cards, several e-money cards including transit IC cards, and PayPay. There is no ATM. The three-bath pass costs ¥800 less than paying separately and has no expiry date. Buy it at any of the three baths.

On Gunma Prefectural Citizens’ Day, October 28, 2026, Gunma residents with ID get half-price entry at all three of the corporation’s day baths.

Mixed bathing on Fridays

Every Friday from 17:30 to 20:00 (last entry 19:30), except some dates, the men’s open-air bath becomes a mixed bath. Everyone covers up. Men are lent a bathing wrap for free and must wear it. Women can rent one for ¥400 or bring a swimsuit. Children need their own swimsuit, as there are no child-size wraps. Women walk in through a door in the partition from the women’s side, so they do not pass through the men’s changing room, and the women’s bath stays women-only. It is popular with couples and families, especially with the evening lighting.

Onsen etiquette

• There is nowhere to wash, so shower first at your inn or at another bath before you come.
• Bathe naked on the men’s and women’s sides except during the Friday mixed session.
• Keep towels out of the water.
• Under Gunma’s public-bath rules, children up to preschool age may go into the opposite-sex bath with a parent.
• The operator leaves babies to parents’ judgement and suggests about six months as the youngest age.
• Pets are not allowed.

Getting there

The bath is at 521-3 Kusatsu, Kusatsu Town, Agatsuma District. Phone 0279-88-6167.

• On foot from town: walk west from the Yubatake through Sainokawara Park, about 12 minutes on a paved path past shrines, statues and steaming pools. The tourism association puts it at about 20 minutes from Kusatsu Onsen bus terminal.
• By car: park free at Tenguyama No. 1 car park (300 spaces), about 6 minutes’ walk from the bath. The operator gives these routes: from the Kan-etsu Expressway Shibukawa-Ikaho IC via Routes 17, 353, 145 and 292, about 60 km; from the Jōshin-etsu Expressway Usui-Karuizawa IC via Routes 18, 146 and 292, about 43 km; or from the Ueda-Sugadaira IC via Route 144, Prefectural Road 59 and Route 292, about 51 km.
• By train and bus: take the JR limited express Kusatsu from Ueno to Naganohara-Kusatsuguchi (about 2 hours 20 minutes), then a JR bus to Kusatsu Onsen (about 25 minutes). Or take the shinkansen to Karuizawa and a Kusakaru Kōtsū or Seibu Kankō bus (about 1 hour 18 minutes to 1 hour 55 minutes).

The Shiga-Kusatsu road (Route 292) over Shibu Pass to Nagano closes in winter. In 2026 it fully reopened on May 29, after the volcanic alert for Kusatsu-Shirane was lowered to level 1.

When to go

The bath is open every day of the year, apart from the short second-Wednesday cleaning closures and occasional maintenance. Winter is the classic time, with snow on the trees around the pool, but opening moves to 9:00 from December to March. In summer it opens at 7:00, and the early hours are the quietest. Autumn weekends and holiday periods are the busiest. Friday evenings are lively because of the mixed session.

Where to stay nearby

You cannot stay at Sainokawara. Kusatsu’s ryokan and hotels cluster around the Yubatake, a short walk from the park, and the operator says discount tickets for its baths are handed out at inns in town and in magazines. See our Kusatsu Onsen page for the free communal baths and the other two day baths on the pass. Elsewhere in Gunma, Ikaho Onsen and Minakami Onsen have their own listings.

Safety notes

• Acidic water: at pH 1.7 the water can sting cuts and irritate sensitive skin. Avoid getting it in your eyes, and rinse off afterwards if you can.
• Metal: remove silver and plated jewellery. Swimsuit fittings may discolour.
• Footing: the path through the park and the rock edges of the pool are wet and can ice over in winter.
• Accessibility: the operator says the bath is not barrier-free.
• Volcano: Kusatsu-Shirane is an active volcano. The bath is in town, far below the zone of about 500 m around the Yugama crater that the Japan Meteorological Agency flagged in May 2026, but check volcanic alerts if you plan to drive the Shiga-Kusatsu road.