PURPOSE: To allow sustention of constant rotational speed of motor by flexing the rotary shaft of the motor with the centrifugal force acting on a weight.

CONSTITUTION: When a weight part 2 fixed eccentrically to the rotary shaft 4 of a motor 5 rotates to actuate a vibration generating part 1, a component of centrifugal force, i.e., thrust A, acts on the rotary shaft 4 thus bringing a shim 7 of the rotary shaft 4 into contact with the end face of a bearing 6. At the same time, a component of force B acts in the direction perpendicular to the thrust A to flex the flexible rotary shaft 4 over the entire length thereof in the longitudinal direction thus rubbing the shim 7 and the bearing 6 strongly and increasing frictional load. Consequently, fluctuation of the speed of motor 5 due to the fluctuation of applying voltage is suppressed and r.p.m. of the motor converges to a predetermined value. This constitution allows sustention of the rotational speed of the motor 5 regardless of the fluctuation of applying voltage.