PURPOSE: To cause the oscillation frequency to approach a set value rapidly without generating hunting, by controlling the count number of a reversible counter by the differential signal between the set value and the oscillation frequency of a voltage variable frequency oscillator VCO and by controlling the VCO by the output of the reversible counter.

CONSTITUTION: The output of a VCO is counterd by a counter N1 and is reset by the clock of a clock pulse generator 1, and the count value is latched in a latch circuit L1. The difference between the set value of a setter S and the value of the latch circuit L1 is operated by a subtractor D and is latched in a latch circuit L2. A counter N2 counts clocks of a clock pulse generator K2 until the counter number coincides with the value of the latch circuit L2, and the counter N2 is reset by the output of a coincidence circuit C. A reversible counter N0 counts clocks of the generator K2 in accordance with the polarity signal of the subtractor D until the output of the coincidence circuit C is issued, and the output of the counter N0 is converted to an analogue value by a D/A converter DA0 to control the VCO, thereby causing the oscillation frequency to approach the set value.