A power transmission system, which transmits a driving force from a rotary actuator to a third joint of a leg link, applies a driving torque to the third joint from an output shaft of the rotary actuator via a driving crank arm, a connecting rod, and a driven crank arm. A translational force (rod transmitting force) generated at the connecting rod in the longitudinal direction thereof is measured on the basis of an output of a force sensor fixed to the connecting rod. With the rod transmitting force or a driving torque applied to the third joint by the rod transmitting force as a controlled variable, the driving force of the rotary actuator is feedback-controlled according to the measured value of the rod transmitting force and a desired value of the controlled variable so that the controlled variable coincides with the desired value.