Hydrangea petiolaris Siebold et Zucc.

Family: Hydrangeaceae
English: Climbing hydrangea
Japanese: ツルアジサイ (蔓紫陽花) [Tsuru-ajisai]

This hydrangea like to creep up trees and bloom high up, where it's difficult to get a good picture. It can be found at elevations from a few hundred meters up into the Central mountains.

References:
九州野山の花 片野田逸朗,2004：南方新社
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）