To provide a joint actuator exhibiting high torque performance in low speed zones and capable of reducing the weight of a bipedal robot by eliminating the need for mounting a decelerator.

In a driving actuator being applied to the joint A at the leg part β of a bipedal robot, a stator 1 for exciting an arbitrary vibration on the circumscription vibration end face S by stacking piezoelectric vibrators 11 and 12 generating natural vibration upon application of a predetermined AC voltage, a rotor 2 for transmitting a vibration excited on the circumscription vibration end face S of the stator 1 as a rotational movement through circumscription contact with the stator 1, and means 5, 61 and 62 for imparting a required preload to the stator 1 and the rotor 2 are provided at the joint A. Based on the vibration excited on the circumscription vibration end face S of the stator 1 by the preloading means 5, 61 and 62, rotational movement is imparted forcibly to the rotor 2 and transmitted to the joint A so that the upper part A1 and lower part A2 of the joint are bent.