On March 16, 1201, while Tomomasa was serving as Obanyaku in Kyoto, his Sanjo Higashinotoin camp was attacked by Nagamochi JO, who called for a decree from the Imperial Court to attack the Kanto region.
建仁元年（1201年）2月3日、大番役で在京しており、関東追討の勅許を求める城長茂に三条東洞院の宿舎を襲われる。