PURPOSE: To obtain a phase detector of simple structure requiring only a single amplifier by connecting the output terminal of an integration amplifier in an integrator with the inverted input terminal of the integration amplifier through a capacitor thereby decreasing the number of active elements.

CONSTITUTION: The phase detector comprises a pulse width modulator 11 and an integrator 12 connected with the post-stage thereof. The pulse width modulator 11 comprises a switch having a control input terminal receiving a pulse subjected to mark space modulation by a phase shift signal. The pulse width modulator 11 also has an amplitude input terminal 13 receiving a signal not subjected to phase shift. The integrator 12 comprises an integration amplifier 16 having output terminal connected through a capacitor C1 with the inverted input terminal of the integration amplifier 16. Since only one amplifier is required, the circuitry can be simplified significantly.