There are quite a few species of Egret (sagi in Japanese) in Japan, but I am not sure which one this is.
There are common, especially in rice paddies and rivers.
They are far more skittish than their cousins the Grey Heron, and if you stop to take a photo even from a distance they will fly off.
So I was really surprised to find one on the road while I was walking in the foothills of Izumo, but it did quickly fly off.
Other posts on Egrets, and the Sagi Mai, incorrectly translated as the Heron Dance can be found here.