I've passed this building hundreds of times, and with its walls of blue corrugated tin I've always presumed it to be some sort of workshop, though I often wondered why it was built in a narrow cleft in the mountainside. This time as I cycled by there were banners out and I realized it was in fact a temple.
On closer examination I was delighted to discover that the temple was built to take advantage of a waterfall cascading down the mountainside.
Underneath the fall was a space for practising misogi, a type of Shugyo (ascetic practise) using water for purification.
There were statues of Fudo Myoo, so in all likliehood this was a Shugendo site before Shugendo was outlawed in early Meiji. Now the temple belongs to one of the newer 20th Century Buddhist sects that have sprung up, many with roots in Nichiren.
Up the mountainside on either side of the falls were large carvings of Fudo Myoo, and the spray from the falls worked like an air conditioner. It was wonderful to discover a delightful place so close to home, and was a reminder to keep exploring!