In the end of January 940, Masakado heard that TAIRA no Sadamori and FUJIWARA no Hidesato were organizing an army, and on February 1, Masakado set off to fight against them, with Harumochi leading a rearguard unit as Assistant Commander.
天慶3年1月下旬、平貞盛・藤原秀郷らが兵を集めているとの報を聞いた将門が2月1日に出陣した際には副将軍として後陣を任された。