Sakura-so (primula japonica)
The plant has rosettes of light green, oblong, toothed leaves. Bears 1 or more whorls of salverform, deep reddish purple to white flowers sometimes with an eye.
Although seed packet grown primulas are grown alot in the winter here, true Japanese primulas sakura-so are seen from April to May and really come into their prime now. Sakura-so look similar to the annual Mericoid primulas, but are a native perennial Primula.
Flowering between April-June, it loves a constantly moist, fertile, neutral to acid soil in light shade, it will tolerate some sun if the soil never dries out.
It is a very easy plant to grow and looks great in a group.
Below is another East Asian primula that can be found easily in Japan: primula chinensis
A link that has a little info on primulas is at this page.