Michimasa, who was designated as Togu gonnosuke (acting vice-minister who serves the imperial prince) in 1011, served Imperial Prince Atsuhira (who later became Emperor Ichijo), and in January 1016, he was appointed as Kurodo no to (Head Chamberlain) when Emperor Goichijo ascended to the throne, and ranked Jusanmi (Junior Third Rank) in the next month.
寛弘八年（1011年）には東宮権亮となって敦成親王（後一条天皇）に仕え、長和五年(1016年)正月、後一条天皇践祚の際には蔵人頭に補せられ、翌月従三位に叙せられた。