Humanoid robots are being expected to do various kinds of jobs in our daily environment in near future. In that case, passing through a door is one of the important functions to be realized by humanoid robots. We analyzed the area that fulfills two conditions: the humanoid robot can reach a doorknob without other collisions with environments, and the humanoid robot can keep stability against the reaction force from the door. The trajectory of the humanoid was then designed by connecting centers of inscribed circles drawn in the area. Effectiveness of the proposed method was experimentally confirmed with the humanoid robot HRP-2.