Rhododendron tashiroi Maxim.

Family: Ericaceae
English: Cherry Azalea
Japanese: サクラツツジ (桜躑躅) [Sakuratsutsuji]

These natives to southern Japan continue to bloom after the sakura trees have finished blossoming, with the season continues into mid-May at higher altitudes. Many areas of old-growth forests may have few giant conifers, but the presence of the large twisting trunks of these azaleas speaks of the age of the forest.

References:
Wada, K. Rhododendron Tashiroi and Its Hybrids. Journal American Rhododendron Society: January, 1965 Vol 19(1) Refer to: http://scholar.lib.vt.edu/ejournals/JARS/v19n1/v19n1-wada.htm .
米倉浩司・梶田忠 (2003-)　「BG Plants 和名－学名インデックス」（YList），http://ylist.info（ 2019年8月14日）