Bolete

Because of the black fibres on the stem (scabers) we can be confident it is a Leccinum species. To get further than that you'd need to consult local literature because our European and American species are unlikely to be the same as the mushrooms found in Japan. It does look like L. scabrum but so do a lot of Leccinum species and like you said the birch is a problem.

thanks for the ID Lenny. It definitely looks like L. scabrum, but I am a bit perplexed that it was growing in an oak forest, not a birch forest. Maybe there were birch trees also present, but I didn't notice them. Any ideas?