In a robot with two or more leg links having ankle joint respectively and pivotably linked to a torso, the robot walks naturally by making the ankle joint of a grounded leg link rotate freely by using passive movement. A controller executes controlling operation of calculating target joint angles of remaining joints other than the ankle joint of the grounded leg link based upon the measured joint angles of the ankle joint of the grounded leg link in the lateral and forward direction. The target joint angles of the remaining joints are calculated so as to satisfy the following condition that a tilting angle of the torso matches a target tilting angle determined based upon the measured joint angle of the ankle joint of the grounded leg link in the forward direction, a cycle period of the idle leg link from lifting to grounding, and a target stride of the idle leg link.