里山の雑木林の中で出合った愛らしいスミレ。シハイスミレ(紫背菫)といいます。
シハイとは「紫背」。葉裏が紫であることからついた名前です。
ちょうど写真の中央に葉裏が写っていますね。

From Satoyama landscape in Shiga prefecture,Japan(7) Viola violacea
"Satoyama (里山) is a Japanese term applied to the border zone or area between mountain foothills and arable flat land. Literally, sato (里) means arable and livable land or home land, and yama (山) means mountain. Satoyama, which have been developed through centuries of small scale agricultural and forestry use, also promise biodiversity if properly maintained by human activities."(http://en.wikipedia.org/wiki/Satoyama)