A frequency synthesizer includes compensation variable capacitance diodes 53 and 54 in a voltage-controlled oscillator 5 in addition to a variable capacitance diode 52 whose DC bias voltage is controlled by a control voltage signal 11 generated by a low-pass filter 3. A monitor circuit 8 monitors the control voltage signal 11 and changes the level of control signals 16 and 17 when the voltage of the control voltage signal 11 goes out of a range within which the capacitance of the variable capacitance diode 52 can be easily changed. A time constant circuit 72 supplies a DC bias voltage to the compensation variable capacitance diode for smoothing out the level change of the control signals 16 and 17 into a slow voltage change so that a locked state is not canceled.