To secure the sufficient number of ultrasonic sound rays even when a rotary encoder of a low resolution advantageous for miniaturizing an ultrasonic scope is used.

An output signal of a reference clock generating means 25 is converted to first and second frequency divided signals c1 and d1 after its frequency is divided by first and second frequency divider circuits 26 and 27. A control circuit 21 triggers a transmitting circuit 22 based upon the first frequency divided signal c1. The phase of the second frequency divided signal d1 is compared with the phase of an A-phase signal a1 of the rotary encoder 15 by a phase comparator circuit 17. The phase compared result of the phase comparator circuit 17 is fed back to keep constant the rotating speed of a motor 14 via a motor control circuit 16. By using such first and second frequency divided signals c1 and d1 for controlling the transmission timing of an ultrasonic vibrator 11 or controlling the rotating speed of the motor 14, even when the rotary encoder of the low resolution is used, the sufficient number of ultrasonic sound rays are secured.