PURPOSE: To stabilize a sweep frequency by providing a reference oscillator having the same frequency as the center frequency separately.

CONSTITUTION: Frequency control oscillator 2 is swept by the sine wave outputted repeatedly from sweeping oscillator 1. In case that, for example, center frequency (a) of the sweep frequency band of swept frequency control oscillator 2 is different from reference frequency (b) of reference oscillator 3, time intervals among trigger pulses (c) outputted successively from trigger pulse generator 5 are different, and a difference in pulse width appears output pulses (d) and (e) of bistable multivibrator 6. This difference is detected by comparison operator 7 and is converted to DC voltage (f) by low-pass filter 8, and after that, the center frequency of the sweep frequency band of frequency control ocsillator 2 is controlled to agree with the reference frequency of reference oscillator 3 by this DC voltage (f).