Hey Tom
There are two species breeding in Hokkaido, Sooty and Asian Brown, As I said I think it is most likely to be an Asian Brown.
Sooty is pretty much restricted to higher altitude forest, and the highest mountains on the Shiretoko are around 1000m, so it is unlikely that they breed on the peninsula or are restricted to the mountain tops. Although other species that are altitudinal breeders in Honshu, breed at much lower elevations in Hokkaido. Asian Brown breeds from sea level to around 2000m above sea level.
Only one species of Ficedula breeds in Japan and that is Narcissus Flycatcher.
It certainly isn't that species. A juvenile would be more greenish-brown above, with a reddish-brown square tail.
Sean
Hiroshima