Although there were provisions for this institution in the Taiho Ritsuryo code, it was implemented slightly later and the description in "Shoku Nihongi" (Chronicle of Japan Continued) shows that the first official certificate was issued in 720.
大宝律令に規定が存在したが、実施はこれより少し遅く『続日本紀』によれば養老4年（720年）に最初の交付が行われたと記されている。