PURPOSE: To put an AFC circuit into secure operation even in the microwave band by detecting a shift in oscillation frequency in the form of the magnitude of reflected electric power from a band pass filter.

CONSTITUTION: The output of oscillator 1 is input to band pass filter 13 through circulator 12. When the oscillation frequency differs from the center frequency of band pass filter 13, reflected electric power is generated. Once the reflected electric power exceeds its criterion, Schmitt trigger circuit 15 is put into operation to reset the control voltage of voltage control oscillator 1 to zero V. The control voltage then rises with time. The oscillation frequency, therefore, starts a sweep. When the oscillation frequency agrees with the center frequency of the band pass filter, the reflected electric power is reduced; and Schmitt trigger circuit 15 is put into operation and sample hold circuit 18 is also put into operation to hold the control voltage of voltage control oscillator 1 as it is.