Fujiyama Onsen is a natural onsen located 1.5 kilometers southeast of Lake Kawaguchi next to Fuji-Q Highland. It is said to boast the largest single bathing room in Japan which is over 1,000 cubic meters and constructed with 12-meter high ceilings using beams of 200-year old wood—all designed to house into which flow nutrient rich water that is good for the skin. It is also a very peaceful onsen, partly due to the use of the ample space, it never feels crowded even when other bathers are around.
The indoors baths are certainly spacious. In the men’s onsen, there is one large rectangular tub, one naturally carbonated tub, a smaller lukewarm bath, and a mizu-buro—a cold-water bath to awaken the senses after a stint in the Finnish sauna. Closer to the showers there are a handful of single “sleeping baths” which jacuzzi-like jet bubbles to massage your body. Outside, there is one long open-air bath with a surrounding ledge made of natural rocks. Unfortunately, Mount Fuji itself is not visible from the open-air baths.
There is a reasonably priced restaurant restaurant/dining area on the 2nd floor and massage chairs, a hammock room, and ganbanyoku elsewhere. It’s not the cheapest of onsen, but certainly the inside baths are among the nicest I’ve visited.