A landing shock absorbing device 18 provided in a foot mechanism 6 of a leg of a robot comprises an inflatable and compressible bag-like member 19 (a variable capacity element) on a bottom face side of the foot mechanism 6. The bag-like member 19 is constructed of an elastic material such as rubber. Air in the atmosphere can flow into and out of the bag-like member 19 by inflow/outflow means 20 provided with a solenoid valve 27 and the like. In a landing state of the foot mechanism 6 and in a state immediately after the foot mechanism shifts from the landing state to a lifting state, the solenoid valve 27 is closed to maintain the bag-like member 19 in a compressed state. Furthermore, during the bag-like member 19 in the inflating state during the lifting state of the foot mechanism 6, by controlling timing when the solenoid valve 27 is switched from a valve opening state to a valve closing state, a height of the bag-like member 19 in a compression direction is controlled to be a height suitable for a gait type of the robot. Thereby, posture stability of the robot can be secured easily while reducing a impact load in the landing motion of the leg of the legged mobile robot, and further, a lightweight configuration can be achieved.