This little man-made grotto/cave with a Fudo Myo is ain Kenzanji Temple located in Hiyoshi, in Hioki on the west coast of the Satsuma peninsula in Kagoshima. It is temple 49 on the 108 temple pilgrimage of Shingon temples on Kyushu.
It isn't an old temple though it has an historic stone pagoda from an older temple that used to exist nearby. The main hall is a portakabin, something I have never seen before. The main hall burned down apparently
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Like many of these small, out of the way, rural temples there is a lot of statuary and such in the grounds, often produced by amateurs.
Eclectic is a word that springs to mind when I visit such places. This was day 36 of my walk along this pilgrimage.... still a few days away from the half way point.