These are flowers of Cornus kousa or more commonly known as kousa dogwood. There're dozens of white and red flowering dogwood trees (Cornus florida) but as far as I know, only a few kousa dogwood trees in my neighborhood. I don't know why.
Kousa dogwood is called "evergreen Buddhist monk" in Japanese by comparing the head (or capitulum) of the flower to the head of a monk of the Hieizan (Mt. Hiei) Enryakuji temple and the four bracts surrounding the capitulum to his hood.
| Kousa dogwood 山法師 |