PURPOSE: To control rotational speed of rotor by providing a piezoelectric body to be excited upon application of a high frequency voltage, a stator comprising a resilient body to be secured to the piezoelectric body, and the rotor to be brought into pressure contact with the stator.

CONSTITUTION: A closed circuit connecting first and second high frequency power supplies 3, 4 is established and voltages having phase difference of 90° are applied from high frequency power supplies 3, 4 onto the piezoelectric body 1a in a stator 1. Consequently, the piezoelectric body 1a is elongated and contracted and standing waves are produced on a resilient body 1b, secured to the piezoelectric body 1a, from first and second high frequency power supplies 3, 4. These two standing waves are then combined to produce a perfect unilateral standing wave. Consequently, a rotor 2 pressure contacting with the stator 1 is rotated, at a constant speed, in the direction reverse from that of the standing wave established on the resilient body 1b.