Emperor Seimu was the 13th emperor of Japan and lived in the 4th century before Emperor Ojin (15th), Emperor Nintoku (16th) and the five kings of Wa (Japan) in what was the first half of the Kofun period.
成務天皇は13代で、応神天皇（15代）仁徳天皇（16代）や倭の五王よりも遡る4世紀のことで、時代でいうと古墳時代の前期にあたる。