I’m pretty certain that this identification is correct, but this butterfly has different markings to the typical ones in all of my butterfly and insect books, which are for mainland Japan. However, the markings are very similar to those for the same species found in Tsushima (between here and Korea): website here.

Lethe diana

Lepidoptera: Nymphalidae
Lethe diana
kuro-hikage

Location: In the forest above Yokoyama Village in Munakata (map).