(troglodytes troglodytes)
Although conventionally simply called a "Wren" by UK and European birders this bird is properly called a Northern Wren or a Winter Wren.
This species is widespread throughout the northern Palearctic and Nearctic with at least 44 sub-species being defined.
It may warrant being split into three full species in which case the Palearctic species would be renamed nannus troglodytes.
The sub-species resident in Japan is t.t. fumigatus which ought to be distinctively dark brown. The bird in this photogrpah does not seem to match the fumigatus description so I do not know what sub-species it might be.