Kashio Onsen is a remarkably unusual hot spring located along the Shio River in Oshika Village, Shimoina District, Nagano Prefecture. Deep in the mountains and far from the sea, it produces naturally salty water with a salt concentration almost comparable to seawater.
This extraordinary hot spring has been closely connected with the daily life and culture of the local community for centuries. Surrounded by quiet mountains and the gentle sound of the river, the simple hot spring settlement offers visitors a peaceful experience far removed from the atmosphere of large resort towns.
The water at Kashio Onsen is classified as a sulfur-containing sodium chloride cold mineral spring, with a source temperature of approximately 14°C.
Its most remarkable characteristic is its salt concentration of approximately 4 percent, giving the water a distinctly salty taste similar to seawater. Although its geological origin remains a subject of interest, the water is not thought to be simply fossil seawater trapped along the Median Tectonic Line because its mineral composition differs from that of seawater.
The exact reason why such salty water emerges in this remote mountain valley has not yet been fully explained. Soaking in mineral-rich water reminiscent of the sea while surrounded by the mountains of Nagano is an experience that can only be found at Kashio Onsen.
Today, Kashio Onsen has two hot spring inns. One of them, Yama-Shiokan, is known as a member of the Japan Association of Secluded Hot Spring Inns, an organization dedicated to preserving and promoting traditional remote hot spring destinations.
Staying at the inn and listening to the sound of the Shio River while relaxing in the hot spring captures the special atmosphere of a true Japanese secret hot spring destination.
Kashio Onsen is also closely associated with a traditional product known as yamashio, or mountain salt. At Yama-Shiokan, traditional salt production was resumed in 1997 following regulatory changes.
The fourth-generation owner revived the traditional method of making salt by heating the mineral-rich spring water over a wood fire and carefully concentrating it until salt crystallizes.
This mountain salt contains very little nigari, the bitter magnesium-rich liquid normally produced during salt making. As a result, it has a gentle character that does not overpower the natural flavors of food and is highly valued for cooking.
Visitors can purchase the salt at the Shio no Sato Local Products Shop, making it a memorable local souvenir and a way to take a taste of Oshika’s unusual mountain culture home with them.
The local products shop is also home to Minoya Tofu Shop, which makes tofu and deep-fried tofu using Heisei Nakao Wase, a specialty soybean variety from Oshika Village.
The tofu has a rich and distinctive soybean flavor despite its simple appearance, and its reputation attracts customers from outside the region as well. The products are also available through mail order, allowing visitors to enjoy the flavors of Oshika even after returning home.
Kashio Onsen has several fascinating legends explaining how its salty water was discovered.
One legend tells that Takeminakata-no-Kami, the deity associated with Suwa Taisha, discovered the spring while hunting deer. He noticed that a deer was licking water that tasted unusually salty, leading to the discovery of the mineral-rich spring.
Another tradition attributes the discovery to Kobo Daishi, Kukai, the famous Buddhist monk who founded the Shingon school of Buddhism. According to the legend, when he visited the area and saw that the villagers were suffering from a lack of salt, he struck the ground with his staff. Salty water then emerged from the earth.
Although these stories belong to local tradition rather than documented history, both legends reflect the importance of salt as a precious resource for people living in this remote mountain region.
The availability of naturally salty water played an important role in the history of Oshika Village. During the Nanboku-cho period in the 14th century, Imperial Prince Muneyoshi is said to have established a base in this area partly because salt was an essential resource for daily life.
In 1875, Senjiro Kurobe, a former samurai of the Tokushima Domain, came to the area in search of rock salt. He carried out excavation work and constructed a large-scale salt-making facility.
Although rock salt was never discovered, the unusual salty spring water continued to attract attention. Even today, the geological reason for its remarkable salinity remains something of a mystery.
The importance of salt can also be seen in the place names and family names that remain in the Kashio area. Names such as Koshio, Oshio, and Manshio are associated with salt.
Salt was indispensable in mountain communities, where it was used for everyday cooking as well as for making simmered dishes, pickles, miso, and soy sauce. In a remote village far from the coast, the naturally occurring salt water was therefore a valuable resource that supported the lives of local people.
The history of Kashio Onsen is consequently not only the story of an unusual hot spring. It is also the story of how a natural resource shaped the food culture, economy, traditions, and everyday life of a mountain community.
Today, Kashio Onsen is known throughout Japan as a secluded hot spring destination with a dedicated following. The surrounding mountains change their appearance with the seasons, while the clear sound of the river creates a soothing atmosphere around the inns.
Visitors can enjoy a relaxing bath while surrounded by the quiet scenery of Oshika Village, an experience quite different from staying at a large urban or coastal resort.
From Ina-Oshima Station on the JR Iida Line, take the Ina Bus Oshika Line. The journey to the nearest Kashio bus stop takes approximately 50 minutes.
From the bus stop, the hot spring area is approximately a 15-minute walk away. This makes Kashio Onsen accessible by public transportation while still retaining the atmosphere of a remote mountain retreat.
Kashio Onsen is a rare mountain hot spring where visitors can experience water as salty as the sea despite being far from the coast. Its unusual geology, traditional mountain salt production, centuries-old legends, and deep connection with local life make it much more than an ordinary hot spring.
A visit here offers an opportunity to slow down, listen to the river, soak in the mineral-rich waters, and discover the remarkable relationship between nature and the people who have lived in the mountains of southern Nagano for generations.