A desired motion determiner of a control unit of a legged mobile robot determines a leg motion parameter specifying the motion trajectory of a distal end of a leg of the robot on the basis of the information on a floor geometry of an environment in which the robot travels and a requirement related to a travel route of the robot, thereby sequentially determining the desired motion of the robot. A floor geometry information output unit which outputs floor geometry information to the desired motion determiner outputs floor geometry information in which a rising surface of a stepped portion of a predetermined type, the contact thereof with a leg of the robot should be avoided, has been shaped into a surface having a gentler slope than an actual rising surface. The desired motion determiner determines the leg motion parameter such that the leg will not come in contact with the stepped portion having the shaped rising surface.