Nagamitsu was an outstanding swordsmith in Bizen Province (Okayama) around the 13th century. This Ken (doubled-edged sword) shows an irregular temper pattern with a Nie type line along the edge of the blade, which is different from his usual work. The figure of the Ken represents the Kamakura period style.
Tradition says that this Ken was treasureed by Tachibana Muneshige, the first lord of the Yanagawa domain. His father, Takahashi Joun gave it to young Muneshige at the age of 15 on the day he was adopted by Bekki Dosetsu with the words, —"if we should ever fight on opposite sides, you should be the head of the army that would kill me, or otherwise commit a suicide, bravely with this sword." Since then, Muneshige always carried it with him to remind him of his father.
This Ken sword was designated as an Important Cultural Property on the 5th of May 1937.