Toyama · Imizu & Shinminato
Kaiō Natural Onsen, Imizu
天然温泉 海王（射水・道の駅カモンパーク新湊隣）
A day spa beside the 'Kamon Park Shinminato' roadside station in Imizu, on Toyama Bay. Its spring is a dense chloride water said to rank among the richest in Japan in dissolved minerals, with several times the free carbon dioxide of shop-bought bath fizzers, so onsen enthusiasts know it as a genuine carbonated chloride spring. The tea-brown, salty, iron-rich water warms you deeply; there are indoor and open-air baths, a sauna and a cold plunge. What stands out is the long day: an early-morning bath from 05:00 through to midnight (23:00 on Sundays), with cheaper early and late rates. Regular admission is ¥970 for adults (¥1,020 weekends and holidays), ¥750/¥780 early or late, ¥520 for children. Closed Mondays (open if a holiday, then closed the next weekday) and January 1. Easy to pair with the Shinminato Bridge, Kaiwo Maru Park and seafood at the Shinminato Kittokito market; about 10 minutes by car from Kosugi IC or 20 minutes by community bus from JR Kosugi Station.
- Price (adult)
- ¥970
- Hours
- 5:00–24:00 (until 1:00 a.m. Fri–Sat; until 23:00 Sun)
- Closed
- Mon (open if a holiday, closed next weekday); Jan 1
- Access
- About 10 min by car from Kosugi IC (Hokuriku Expressway); about 20 min by Imizu community bus (Shinminato–Kosugi line) from Kosugi Station; about 10 min by car from Kosugi (Ainokaze Toyama Railway) or Higashi-Shinminato (Manyōsen)
- Water type
- Simple spring
- Tattoos
- Ask ahead
- Admission
- Prices revised 21 April 2026: all baths including the 'Shin' raw-source rotenburo — adults ¥970 weekdays, ¥1,020 weekends and holidays; elementary ¥520; ages 3+ ¥250. Plain and bicarbonate baths only ¥800 / ¥850. Early bath (5–9 a.m.) ¥750 on weekdays (¥220 surcharge if you stay past 9). Late-night rate abolished. Cash and PayPay etc. only — no credit cards. Shampoo provided; bring or buy a towel; dryer ¥100 per 5 min; free foot bath
- Baths
- The heart of the place is the surcharge zone: the 'Shin' raw-source rotenburo and a 'pure raw' source tub with no dilution or heating. Also a nano-bicarbonate bath, jacuzzi, artificial radium bath of Hokutolite stone, rotating bath, iki sauna (about 90°C, 9 people) plus an outdoor CUBERU sauna on the men's side, a groundwater cold plunge (about 15°C) with bucket shower, and an outdoor cooling area. Free mud packs in the women's bath. Restaurant, lounge with 2,500 manga, massage and Korean scrub, paid private rooms, Wi-Fi
- Water
- Sodium chloride strong-salt spring (hypertonic, neutral, high-temperature), 46.7°C, pH 7.0, 320 L/min at source, 29.55 g/kg dissolved solids. Ochre-coloured brine from 1,200 m down, billed as roughly the fourth most mineral-rich in Japan after Arima, Yoshikawa and Shin-Appi. The source zone runs 100% free-flowing with no dilution or heating; strongly salty and it stings cuts. Indicated for neuralgia, joint pain, frozen shoulder, bruises, chronic digestive complaints, poor circulation and fatigue
- Parking & access
- Free parking for about 90 cars. Directly behind the 'Michi-no-eki Kamon Park Shinminato' roadside station on Route 8. About 20 min by Imizu City 'Kitokito' community bus from Kosugi or Etchu-Daimon Station (Ainokaze Toyama Railway) to 'Kagamimiya-kita'. About 10 min from Kosugi IC on the Hokuriku Expressway; about 25 min by car from central Toyama
- Phone
- 0766-82-7777
- Official site
- kaiou-onsen.com ↗
- Open in Google Maps
- Google Maps ↗
Essentials
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Why go
- Mineral-dense carbonated chloride spring
- 5 am–midnight; cheaper early and late
- Next to the roadside station; Shinminato seafood
Japan's fourth-richest brine, open from 5 a.m.
Tennen Onsen Kaio is a large day-spa behind the 'Kamon Park Shinminato' roadside station on Route 8 in Imizu, Toyama, affectionately nicknamed 'the strongest and best, Kaio' by the prefecture's onsen and sauna fans. Its source, from 1,200 m down, is a sodium chloride strong-salt spring with 29.55 g/kg of dissolved solids — billed as roughly Japan's fourth most mineral-rich after Arima, Yoshikawa and Shin-Appi — and this ochre brine is used 100% free-flowing with no dilution or heating. Inside, the décor is crammed like an old-fashioned sweet shop, and with hours from 5 a.m. to midnight (1 a.m. Fri–Sat) it doubles as a morning bath for long-haul drivers and commuters.
The 'Shin' source zone, sauna and groundwater plunge
The baths are split in two. The base-price zone has a plain bath, a nano-bicarbonate bath said to aid circulation, a jacuzzi and an artificial radium bath of Taiwanese Hokutolite (an octopus slide in the women's bath and a crab sculpture in the men's keep children happy); for ¥170 more, the source zone offers the 'Shin' raw-source rotenburo and a 'pure raw' tub filled straight from the spring. It is salty enough to sting cuts, keeps you warm for hours and, with a generous overflow, is always fresh. The sauna is an iki sauna at about 90°C (the men's side adds an outdoor CUBERU sauna), and the cold plunge runs with drinkable Toyama spring water; pull the rope and a bucket of it crashes over your head — a house speciality. Free mud packs in the women's bath, and the soy-milk soft-serve (¥400) afterwards is the ritual.
Practical details and tips
Hours are 5:00–24:00 (until 1:00 a.m. Fri–Sat, 23:00 Sun); closed Mondays (open if a holiday, closing the next weekday) and 1 January. Since the 21 April 2026 revision, adults pay ¥970 weekdays / ¥1,020 weekends with the source zone, ¥800 / ¥850 for the plain and bicarbonate baths only, and ¥750 on weekdays for the 5–9 a.m. early bath; the late-night discount is gone. Payment is cash or PayPay and the like — no credit cards, so be prepared. Bring or buy a towel; shampoo is provided. It is popular and weekend evenings are packed, so early mornings and weekday afternoons are more comfortable. It pairs well with a half-day of white-shrimp dishes at the roadside station next door and a stroll along the Uchikawa canal in Shinminato; 25 minutes by car from Toyama Station.
Before you go
- No official tattoo policy is published — ask at reception.
- Last entry is 30–60 minutes before closing.
Features
Sauna & wellness
Price
Who with
Amenities
✔ Verified operating: 2026-09 Operating status and rates cross-checked against the official site and onsen guides (full source notes in Japanese on the JA page)
Prices and hours change. Check official sources before visiting.
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