Could be a species of evergreen oak?

Well, there must be hundreds of evergreen tree species that can grow in Japan. Are the undersides of the leaves a grayish color? Can you get a good pic of the undersides? Also, need a closeup of a twig to see bud arragement and general twig size. Are there any acorns, or fruit or spent flower stalks? That's what you'd probably need to ID it.

I would say it does not look like most of the evergreen oaks in the Cyclobalanopsis section I'm familar with, such as Q. myrsinfolia, acuta, glauca, phillyreoides, or salicina. Quercus typically have a cluster of dormant buds on the twig tip.

The prominant veins remind me of a Beech (Fagus). But are there evergreen Beeches in Japan?

Could possibly be in the Lithocarpus, Nothofagus or Castenopsis genera. I am growing C. cuspidata but mine has a metallic sheen to the leaf undersides and does not have prominant veins.