PURPOSE: To avoid the effect of temperature change by inserting an amplifier, a filter and a waveform converter having nearly the same characteristic as that of an amplifier and a filter at the input side and an output side waveform converter between a voltage controlled oscillator and a phase comparator in series.

CONSTITUTION: A base band signal is subject to full wave rectification 5, amplified (6) and detected (7) and the result is inputted to one input of a phase comparator 8. A low frequency component being an output of the comparator 8 is filtered (9) and subject to error amplification 10 to drive a voltage controlled oscillator 11. An amplifier 16, a filter 17 having nearly the same characteristic as that of the input side amplifier 6 and the filter 7 and a waveform converter 12' having nearly the same characteristic as that of the output side waveform converter 12 are connected in series between the oscillator 11 and the other input of the phase comparator 8. Let the phase change of the amplifier 6, the filter 7 and the waveform converter 12 due to temperature be 1∼3, the phase 1+2 is canceled by the phase comparator 8 and -3 appearing at the voltage controlled oscillator 11 is canceled by the phase +3 of the waveform converter 12. Thus, the effect of the temperature change is hardly received.