Gentiana zollingeri Fawc.

Family: Gentianaceae
Japanese: フデリンドウ (筆竜胆) [Fuderindō]

This little gentian is a only an inch across. If it's not blooming in the morning, come back in the afternoon. In my experience, it seems to like shadier locations than G. squarrosa, and the stalk grows longer accordingly, although they can be short as well. Now, tell me why I've only seen white flowers!

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）