Rhododendron yakuinsulare Masam.

Family: Ericaceae
Japanese: ヤクシマヤマツツジ (屋久島山躑躅) [Yakushima Yamatsutsuji]

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. Flashes of bright pink on the mountain side and low to middle elevations. The color is similar to that of R. indicum.

References:
Niihara, S. Rhododendron yakuinsulare. The Rhododendron, Camellia & Magnolia Group Bulletin. November 2008. (98)p.4-5. Refer to: http://www.rhodogroup-rhs.org/docs/bulletins/RCMG%20Bulletin%2098.pdf
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）