This onsen is the biggest one in Yamanashi, and open-air onsen is so popular that is natural hot spring contained FFC ceramic and it is said that good for recovering the tiredness and for beauty. "FFC Healthy hot water" has the similar function of ancient ocean, the material has a power of change things into new which Ferrous Ferric Chloride can effect through the water.
You can enjoy wain hot spring at the observatory spa on the 7th floor. You can enjoy the smell and enjoy the view of Kofu Basin, and you can see Mt. Fuji above the Misaka Mountains.