In those days, Ganryu-jima Island (located in the Kanmon Strait) was called Funa-shima Island (literally, 'boat island') and belonged to the Kokura Domain of Buzen Province (at present, it belongs to Shimonoseki City, Yamaguchi Prefecture); at the island, Musashi fought with a heihoka, Kojiro SASAKI in the Keicho era (1596-1615).
これは慶長年間に豊前国小倉藩領（現在は山口県下関市域）の舟島（関門海峡に浮かぶ巌流島）で、佐々木小次郎なる兵法者と戦ったとされるものである。