PURPOSE: To realize a wide-angle phase modulation by dividing both the ultrasonic oscillation frequency signal and the ultrasonic reception signal by the divider and then supplying these signals to the phase comparator.

CONSTITUTION: The oscillation signal of ultrasonic oscillation circuit 6 is 1/N-divided by divider 13 to be supplied to voltage control phase deviation circuit 7. Thus the phase deviation angle of circuit 7 is lowered down to 1/N. At the same time, the phase synchronous loop is formed with phase comparator 9, circuits 6 and 7 plus loop filter 10, and the synchronization is secured for the output of circuit 7 with the ultrasonic reception signal to deliver the phase modulation output through output 1. On the other hand, the ultrasonic wave sent from ultrasonic oscillator 4 receives the phase modulation via the Karman vortex 3 and is then received at ultrasonic receiver 5 to be supplied to phase comparator 9 after 1/N dividion via divider 12. The phase difference between the 1/N division waves equals to N times as much as the phase difference of the basic frequency. As a result, the phase range to be phase- demodulated in a high linearity can be expanded to N-fold to secure the output through output 2 and via LPF11.