Akayu Onsen

Akayu Onsen is a remote hot spring (onsen) and the single mountain lodge that operates it, Yamaguchikan, located in Mikuni, Yuzawa Town, Minamiuonuma District, Niigata Prefecture, Japan. It sits in the uppermost reaches of the Shibumi River (Shimizu River) valley and is reachable only on foot, making it a classic Japanese "hitou" (hidden hot spring). The site is sometimes treated as part of the Echigo-Yuzawa Onsenkyo hot spring area. The spring was known for generations, but the first hot spring inn was built in 1897 (Meiji 30). Ownership changed several times; in 1919 the ancestor of the current operators opened Yamaguchikan, and the Yamaguchi family continues to run it today. The lodge was rebuilt in a lodge style in 1989. Yamaguchikan operates as a mountain hut midway along the Mount Naeba climbing trail. It offers three self-flowing open-air baths fed by separate sources: Aoyu ("blue water"), Tamagoyu ("egg water"), and Yakushiyu. Aoyu is the furthest downstream and is reserved for women during the day; Tamagoyu and Yakushiyu are mixed-gender baths. There is also a spot upstream of the lodge where hot water wells up when bathers dig into the riverbed, allowing a wild "noyu" (open-air river bath) experience. The inn serves overnight guests and day-trippers, and functions as a base for climbers heading to Mount Naeba and Mount Akakura. The setting is deliberately off-grid: although the lodge has its own generator, it is used sparingly to conserve gasoline, and lamps are normally used at night. Mobile phone signals do not reach the valley, and even satellite phone reception is unavailable. The inn closes in winter, from November to April, and typically opens before the May holidays. Access is by car to a parking area at the end of the Akayu forest road, followed by roughly two hours on foot along the trail. The area (Yuzawa Town Mikuni Aza Akayu) is designated by Japan Post as a "transportation-difficult area," so mail cannot be sent there from outside. The official website provides seasonal updates, trail conditions, and visitor guidance, including COVID-era requests for guests to bring masks, sanitizer, and towels, and to take their garbage home.

The hot spring at Akayu was known for a long time, but the first hot spring inn was built in 1897 (Meiji 30). The management of the spring changed hands several times. Fukuhara Hikosaburo of Ojiya, Niigata Prefecture, began operating it, and later management passed to his relative Tanaka Sohei and then to Yamaguchi Chiyomatsu, whose descendants run Yamaguchikan today. In 1919, the ancestor of the current operators opened Yamaguchikan. In earlier times, outside the normal operating season, the lodge sometimes served as a hunters' hut in early spring; today it customarily opens before the May holidays. In 1989 (Heisei 1), the building was rebuilt in a lodge style, becoming a modern hot spring inn. The inn has continued to operate as a mountain hut midway along the Mount Naeba trail, with three self-flowing open-air baths fed by separate sources. The official website records seasonal updates, including trail maintenance and the resumption of helicopter supply lifts in May, and notes that the lodge remains open with infection-prevention measures in place.

• 2026

  Official website updated with 2026 season trail and operation information.

• 1989

  Yamaguchikan rebuilt in a lodge style.

• 1919

  Ancestor of the current operators opens Yamaguchikan.

• 1897

  First hot spring inn built at Akayu Onsen.

A secluded, off-grid, walk-in-only hidden hot spring (hitou) and mountain lodge, valued for its untouched natural setting and self-flowing springs.

• Yamaguchikan lodge

  The single inn at Akayu Onsen, operating as a mountain hut midway along the Mount Naeba trail.

• Aoyu (blue water) open-air bath

  The furthest downstream bath, with slightly blue-tinted water; reserved for women during the day.

• Tamagoyu (egg water) mixed open-air bath

  A mixed-gender open-air bath with yellow-brown water and three tubs at different temperatures.

• Yakushiyu semi-open-air bath

  A mixed-gender semi-open-air bath with pale yellow-brown water; formerly an indoor bath.

• Riverbed noyu

  A spot upstream of the lodge where hot water wells up when the riverbed is dug, allowing wild open-air bathing.

1. 1赤湯温泉 山口館 公式ウェブサイトakayunaebasan.sakura.ne.jp
2. 2赤湯温泉 (新潟県) - Wikipediaja.wikipedia.org
3. 3Akayu Onsen - Wikidatawikidata.org