Mamashita Onsen

Located in the Setoyama district near the center of Niijima Island in the Izu Islands, about a 10-minute walk from Niijima Port, this village-run day-use hot spring facility opened in 1983 as a place for local residents' training, rest, and health promotion; it was renovated in fiscal 1995 and gained its now-famous sand bath in fiscal 2002. Part of the building uses kokaseki, a volcanic stone unique to Niijima.

The water is a sodium chloride spring (hypertonic, neutral, high-temperature) with a source temperature of 79.8°C and a nearly neutral pH of 6.86. It carries a pronounced seawater-like saltiness and is rich in metasilicic acid, prized for its skin-softening effect. The high mineral concentration keeps bathers warm long after leaving the water, and the bath is known for preventing post-bath chill.

The facility features an indoor bath, an open-air bath, and a sauna; the open-air bath looks out over Mamashita Beach toward Shikinejima and Kozushima. Next door is a sand bath using white sand from Hafuura Beach (separate fee). Opening hours are 13:00 to 21:00 (last entry 20:30), closed every Wednesday, with admission of 300 yen for adults, 150 yen for elementary school children, and 200 yen for ages 70 and up; cash only.