It is believed that the sword was offered by Susanoo no mikoto to Amaterasu Omikami and then handed down to the Imperial family along with the Yata no Kagami (mirror) as shintai (objects of worship housed in a Shinto shrine and believed to contain the spirit of a deity) of Amaterasu.
剣は須佐之男命から天照大神に奉納され、天皇家に天照大神の神体として八咫鏡とともに手渡された事になっている。