After that, a rule was made before the eighth century for an emperor to send a mission once every 20 years as described in the book written by the Tang priest Yuiken and a mission was sent approximately every dozen to two dozen years.
その後、唐僧維躅（ゆいけん）の書に見える「二十年一来」（20年に1度）の朝貢が8世紀ごろまでに規定化され、およそ十数年から二十数年の間隔で遣唐使の派遣が行われた。