Kaiseiyu in Kamata: A Black-Water Bathhouse with Fish in the Wall
Try Ota's dark natural spring at Kaiseiyu in Kamata, with a carbonated black-water bath and fish tank by the baths. Adult entry is ¥600.
At Kaiseiyu, the memorable detail is not a rooftop view or a huge sauna. It is the contrast between dark water, the pale bathing room and fish moving behind glass beside the baths. The bathhouse in Nishi-Kamata makes a practical evening stop if you want to try Ota’s natural black spring without setting aside half a day.
Three ways to use the black spring
Kaiseiyu’s appeal is the range of baths built around the same dark spring water. There is a regular black-water bath, a black-water bath with added artificial carbonation, and a cooler black-water pool. A silk bath and a clear-water bath round out the compact selection. The carbonated pool is reported at roughly 39–39.5°C, a gentler temperature for anyone who prefers not to stay long in a hotter bath.
The cool black-water pool gives you a hot-to-cool option without a sauna. The operator does not describe it as source-fed or free-flowing, so it is not a documented natural-spring pool. If you want both a sauna and a source-fed cold bath, Hasunuma Onsen is a better fit.
Kaiseiyu’s spring is described as a sodium-bicarbonate cold mineral spring. The water is heated for bathing; older analysis figures online should not be treated as a current 2026 measurement. As with other Ota black-water baths, its colour comes from naturally occurring organic material in the groundwater.
What it costs and when to go
Kaiseiyu follows Tokyo’s public-bath rate effective 1 August 2026. It is a pay-as-you-go sento; there is no robe, lounge or extended-stay package.
| Item | Price |
|---|---|
| Adult admission | ¥600 |
| Child admission (age 6 to under 12) | ¥200 |
| Child admission (under 6) | ¥100 |
| Bare-essentials set (shampoo, conditioner, soap, toothbrush and towel) | ¥220 |
| Adult visit with the set | About ¥820 |
Kaiseiyu is open 3 p.m.–11:30 p.m. and closes on Fridays. Older pages may show a later closing time; plan around 11:30 p.m. rather than counting on a midnight arrival. From JR or Tokyu Kamata Station, allow about seven to eight minutes on foot. There is limited parking, but the station walk is usually the more dependable option.
One older tourism listing mentions a sauna, but more recent facility information identifies Kaiseiyu as having no sauna. Choose it for the black-water baths, not for a sauna circuit. For a hotter bath and an old-school dining hall, Kamata Onsen has a different feel; the four Kamata black-water baths compared lays out the trade-offs.
If you arrive with luggage, leave larger bags at your hotel or a station locker. Kaiseiyu is a local bathhouse rather than an airport spa with dedicated baggage facilities. The fish tank is a charming detail, but children still need close adult supervision around the baths.
Sources
- [Ota City Tourism Association: Kaiseiyu, hours and access](https://www.o-2.jp/contents/%E6%94%B9%E6%AD%A3%E6%B9%AF/) - [Ota City Tourism Association: recent Nishi-Kamata bathhouse feature](https://www.o-2.jp/feature/nishikamata-sento-malatang/) - [Tokyo Sento Association: Kaiseiyu black-water and carbonation feature](https://www.1010.or.jp/mag-tokyosento-kaiseiyu/) - [Tokyu: bathhouses around Kamata Station](https://www.tokyu.co.jp/area/kamata/article/arti-01JTJAVGSG1S25F8C6W3BPKYDA/) - [Tokyo Metropolitan Government: public-bath admission rate effective 1 August 2026](https://www.metro.tokyo.lg.jp/information/press/2026/07/2026072121) Checked: 2026-09-27Tags: Kaiseiyu, Kamata onsen, Ota City, Tokyo black water, Tokyo sento
