This Bird is Mejiro, which is a Japanese White-eye.
I have thought this bird is Uguisu, which is Japanese Bush Warbler till check with Wikipedia.
Many of Japanese confused Mejiro and Uguisu.
Because Japanese called Uguisu-Iro, which is the name of color, is same as Mejiro feather color.
The Japanese White-eye starts singing from early spring on or by the plum trees.