PURPOSE: To operate efficiently an ultrasonic wave device by feeding back only a signal of a frequency within a preset upper/lower limit to a voltage controlled oscillator circuit so as to track only an object frequency without causing a spurious frequency thereby keeping the operating efficiency of an ultrasonic wave oscillator in the best condition.

CONSTITUTION: A frequency upper/lower limit circuit 6 is provided between a low pass filter 3 and a voltage controlled oscillator 1 in a PLL oscillator circuit feeding back an output signal of a phase comparator circuit 2 is fed back to the voltage controlled oscillator circuit 1 via the low pass filter 3. Thus, the transmission of the oscillation frequency to a spurious frequency is prevented and only the object frequency is to be tracked. That is, if a nearby spurious frequency is larger than the object frequency as shown in waveform (m) because of the change in the load condition, the upper/lower limit of the frequency of the output signal from the low pass filter 3 is limited and only a signal within a prescribed range as shown in a signal (e) passes through as an input to the voltage controlled oscillation circuit. Thus, the frequency is not shifted to the spurious frequency under any condition in the phase comparison by the phase comparator 2 and only the object frequency is tracked.