It is one of the flowers that starts to bloom in spring, shortly after the cherry blossoms.
It is a plant of the Asteraceae family native to North America, and can grow to about waist height, which I think is relatively tall for a flower that blooms at this time of year.
It may have survived the winter in the form of a rosette and then grown rapidly in the spring.
The flower shape is typical of a plant in the Asteraceae family, and like sunflowers and dandelions, it is made up of many small flowers gathered together to form one flower.
It is a close relative of the daisy family, which I introduced on Instagram in June last year, and looks very similar.
The name of the flower for the Japanese buttercup is "Shion" and the name of the flower for the daisy is "Jeon".
Although it may be confusing, they are written in kanji as "Shunshokunin" and "Himejokunin" respectively.
There are apparently a few ways to tell them apart, and you can sometimes tell by their appearance, but I suspect that it is a dandelion because: the dandelion flowers earlier, blooming from early April; the buds are drooping; the petals can be purple; the leaves have no stalks; and the stems are hollow.However, after May when the dandelions start to bloom, there are many similar plants and it can be difficult to tell the difference.
(It seems there may also be some kind of mutant species, but I don't know anything about that...)
It is a plant that commonly grows around the Yoyogi office and is very familiar to me as I have seen it since I was a child, but at the same time, along with the fleabane, it is listed as one of the "100 worst invasive alien species in Japan."
MURAKAMI Isao