Abstract: The rotation of a motor is converted into a swinging movement of an ultrasonic transducer by a conversion mechanism. The relationship between a rotation angle .theta. of the motor and a swinging angle .eta. of the ultrasonic transducer can be expressed by .eta.=tan.sup.-1 (sin.theta.). The rotation of the motor is detected by a rotation detector of a reluctance type having an eccentric rotor. A signal having a nonlinear relationship with respect to an actual rotation angle .theta. of the motor, which can be expressed by the following equation, is supplied to a motor driver from the rotation detector:.theta..sub.D =.theta.+K sin 2.theta..A dummy rotation angular velocity .omega..sub.D which is a rate of change with respect to a time of a dummy rotation angle .theta..sub.D is obtained by the driver. The motor is servo-driven with reference to a deviation value between the dummy rotation angular velocity .omega..sub.D and a velocity setting value .omega..sub.M.