A motion state evaluation apparatus of a legged mobile robot uses virtual surfaces (S3a, S2a, S2b) to approximate a plurality of surfaces to be contacted (FL, WL1, WL2) in an operating environment of the robot (1), and calculates translational forces (required virtual surface translational forces) to be applied from virtual surfaces (S3a, S2a, S2b) to the robot (1) in order to implement a compensating total translational external force related to a translational motion of the robot (1). The motion state evaluation apparatus evaluates the motion state of the robot on the basis of the required virtual surface translational forces.