PURPOSE: To supply the voltage having an amplitude corresponding to the ratio of phase deviation between a reference signal and an input signal, by using a frequency converter which converts the reference signal into an integer multiple as much as the frequency of the input signal.

CONSTITUTION: In case an input signal is set at 4kHz and the detected phase deviation step is set twice, a reference signal 11 of 60kHz applied to an input terminal 1 undergoes the 12-multiplication through a frequency converter 2 and is converted into a signal of 720kHz. This signal is multiplied by a gate circuit 4 together with an input signal 13 applied to the circuit 4 through an input terminal 3. Then a converted pulse is convoluted into 1/2 pulse width of the input signal to obtain a signal 14. While the reference signal supplied from the terminal 1 is applied also to a frequency converter 5 to undergo 15 divisions and converted into a reference signal 15 of 4kHz. The pulse signal produced in response to the phase deviation between the signals 11 and 13 is applied to a counter 7. Then the voltage having an amplitude proportional to the number of applied pulses is delivered from an output terminal 9 of a D/A converter 8.