Nangakuji Temple is one of the Tsuruoka-area temples associated with sokushinbutsu, the severe practice in which monks slowly prepared their bodies for self-mummification. The mummy preserved here is Tetsuryukai, a monk said to have entered religious life after violence in his youth.
It is not a casual sight, and it should be approached with respect rather than curiosity alone. But it is also one of the experiences that makes this part of Yamagata feel very different from Japan's more familiar temple routes.