An ultrasonic motor has first and second elastically vibrating rotatable means and third rotatable means interposed between the first and second rotatable means. The first and second rotatable means, which are made elastic disk members, generate mechanical vibrations by means of an electrostrictive effect of piezoelectric elements cooperating therewith. Locking means is provided in association with the first to third rotatable means so as to selectively engage with the first to third rotatable means, whereby allowing the rotation of at least one of the first to third rotatable means.