Cardiocrinum cordatum

Family: Liliaceae
Japanese: ウバユリ

I'm told this is a super-common plant in other parts of Japan, and Wikipedia lists several references indicating that it is naturalized in Maryland, USA. However, it seems that the deer have a penchant for it, and I only know of this one specimen in Kosugidani. In 2015-2016, the deer population in this particular area shrank, and it bloomed beautifully for about a week.

References:
屋久島の植物 川原勝征,初島住彦(監修),1995：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（ 2016年10月14日）.