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Hoshino Resorts opens Kai Kusatsu as it expands the Kai onsen ryokan brand

01 Jul 2026

Kai Kusatsu, an onsen ryokan property has launched in the famous Japanese hot spring destination, Kusatsu Onsen in Gunma Prefecture.

Parent company Hoshino Resorts launched the resort to coincide with the 15th anniversary of the Kai ryokan brand – which is set to grow to a portfolio of 30 properties across Japan by 2030.

Read our interview in Spa Business with Yoshiharu Hoshino, CEO of Hoshino Resorts, about his plans to pursue wellness investment opportunities in Japan and further afield here.

Kai Kusatsu

Kai Kusatsu has opened as a forest resort with 94 rooms, 18 of which have private open-air baths. The estate measures 11,844sq m, with buildings occupying 3,701sq m. Read our earlier development story here.

Tatsuro Sasaki Architects and Associates has designed a nature-rich and peaceful retreat that blends into the surrounding landscape of rock faces and vegetation that thrive in the region’s highly acidic environment. 

Each Kai property features room elements that reflect regional culture and in this case, Gunma’s heritage silk production is on full display. Textile artist Reiko Sudo’s Kiryu weaving techniques are on display in the form of lampshades and custom cushions.

Construction firm Maeda Corporation has developed a private passage for guests that connects the property to the vibrant hot spring town, further down the hill. Landscape design is by Sea Bass. 

Kai Kusatsu has introduced a walking programme for guests called the Kai Kusatsu Strolling Kit. To elevate local exploration, guests are encouraged to walk through Kusatsu in lightweight kimonos known as yukatas with a square cloth furoshiki bag. The kit includes a pass for three of the onsen town’s public baths, followed by a café break. A guidebook is also on offer, titled Recommendations for Strolling Kusatsu, with insider tips and local spots to visit.

Toshihiro Ishijima, GM of Kai Kusatsu, told Spa Business, "Kusatsu Onsen is one of Japan’s three most famous hot spring districts, making it a premier hot spring destination that attracts a vast number of visitors throughout the year. For Kai, opening a property in Kusatsu Onsen holds immense significance.

“Kusatsu Onsen is visited by many international tourists. We look forward to delivering an accommodation experience to global guests that allows them to connect with Kusatsu's mountainous landscapes, rich nature, abundant hot spring waters and local culture."

Kai Kustatsu’s bathhouse

Kai Kusatsu’s bathhouse features three separate tubs with water from two separate sources, the Bandaiko source and the Nishinogawara source at varying temperatures.

The Bandaiko is a highly acidic, sulfate hot spring with high levels of chloride. This is the source that feeds the public Kusatsu Onsen baths and is known for the highest natural release volume in Japan.

The Nishinogawara source is a rarer, less acidic spring whose water is used by only a few facilities.

After using the baths, there is a relaxation area called the Fireplace Lounge, which is divided into four distinct zones. A fireplace stands in the centre, and there is a curated collection of books on the culture and nature of Gunma and Kusatsu for guests to enjoy.

The property offers two dining options: the main restaurant Joshu Hoden Kaiseki and food served at a restaurant called Soba Kappo Sai. Soba Kappo Sai is the Kai brand’s first venue open to the public during the day to non-hotel guests.

Kai bathing programme

The Kai brand is currently offering a medically supervised bathing experience, “cool-down onsen soak”, at a number of its onsen properties. It is using this programme to promote onsen bathing as a tool for recovery in response to record-breaking extreme heat across Japan last year.

Future Kai properties

Hoshino Resorts will open various Kai properties in Japan in the coming years.

Kai Miyajima – Opening 23 July, this will be Hoshino Resort’s first venture into Hiroshima Prefecture. The hotel’s rooms will all face the ocean and the bathing experiences will include the revival of a traditional Setouchi Ancient Sauna experience that dates back to the Kamakura period (1185-1333).

Kai Matsumoto – Reopening on 4 August in Nagano Prefecture, this property has undergone a major renovation to create a more luxurious and contemporary onsen experience. The bathhouse will have eight different hot spring baths. The hotel will also offer local wines, music-inspired experiences and Italian-inspired dining instead of traditional kaiseki cuisine. This is the first time the Kai brand will introduce a European dining option to one of its properties.

Kai Zao – Opening 15 October in Yamagata Prefecture, this site is located in the Zao mountain range. The immersive wellness journey is designed around a circular sequence of experiences, which include highly acidic hot spring baths, sauna sessions, indoor cooling and relaxation spaces.

Kai Ureshino – Due to open in Q3 2027, this property will mark the Kai brand’s entry to Saga Prefecture for the first time. It will be located in one of Japan’s most celebrated hot spring destinations.

The brand has plans to educate visitors on the history, wellness and regional customs associated with Japan’s hot spring traditions via an 'ABCs of Onsen Culture' programme across all of its properties.