An external force target generating device of a legged mobile robot uses virtual surfaces S3a, S2a, and S2b to approximate a plurality of surfaces to be contacted FL, WL1, and WL2 in an operating environment of a robot 1, and determines provisional values of translational forces (required virtual surface translational forces) to be applied from the virtual surfaces S3a, S2a, and S2b to the robot 1 in order to implement a compensation total translational external force related to a translational motion of the robot 1. Further, in order to implement a compensation total moment external force related to a rotational motion of the robot 1, the external force target generating device determines moment compensation amounts to be combined with the provisional values of the required virtual surface translational forces and then determines the desired external forces to be applied from the surfaces to be contacted to the robot 1 and the desired external force action points on the basis of the combinations of the provisional values of the required virtual surface translational forces and the moment compensation amounts.