Lilium longiflorum Thunb.

Family: Liliaceae
English: Easter Lily
Japanese: テッポウユリ (鉄砲百合) [Teppō Yuri]

Yakushima is the northern limit of Easter lily's natural range, and you can see them blooming along the coasts, as well as the invasive L. formosanum　with thinner and more plentiful leaves and hybrid L. x formolongo, both of which bloom slightly later.

References:
新版　屋久島の植物 川原勝征,初島住彦(監修),2001：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）