The Japanese Wagtail (Motacilla grandis) was formerly treated as conspecific with the White Wagtail (Motacilla alba), which also occurs in Japan.
When you see it you will notice that the Japanese Wagtail is slightly bigger than the White Wagtail.

1st, 2nd & 3rd photo: Kanazawa, Ishikawa Prefecture, Japan, 25 September 2018
4th & 5th photo: Shirakawago, Gifu Prefecture, Japan, 26 September 2018