Gentiana squarrosa Ledeb.

Family: Gentianaceae
Japanese: コケリンドウ (苔竜胆) [Kokerindō]

This little gentian is a only about an inch across. You can find it in sunny areas (which enourage short stalks) at altitudes from about 800m to 1500m I'd say. If it's not blooming in the morning, come back in the afternoon. (The etymology is driving me nuts. koke rindou is G. squarrosa and fude rindou is G. zollingeri, but Yakushima koke rindou is sometimes considered a subspecies of G. zollingeri. It seems that the leaves of G. squarrosa are bigger near the bottom of the plant, whereas the leaves of G. zollingeri aren't particularly larger near the bottom. )

References:
房総の山のフィールド・ミュージアム ニュース No.448　2007/03/28 http://www.chiba-muse.or.jp/NATURAL/special/yama/news/2007/070328gentiana.htm
Panda. 花の日記　2009.04.16　ハルリンドウとフデリンドウとコケリンドウと　http://nannjyamonnjya.blog68.fc2.com/blog-entry-662.html
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）