PURPOSE: To prevent deterioration of phase detection accuracy, which is caused by a delay time of an element, etc., by directly detecting a phase shift of an input signal and a comparison signal, by an output of a monostable multivibrating circuit which is set by the input signal, and the comparison signal.

CONSTITUTION: A monostable multivibrator 11 is set for a constant time by an input signal, and generates a signal A. If the signal A annihilates before a comparison signal B is generated, a flip-flop 12 is set from fall of the signal A till generation of the signal B, and forms a signal X showing that phase is proceeding. On the other hand, if the comparison signal is generated before the signal A annihilates, a flip-flop 13 is set from rise of the signal B till fall of the signal A, and forms a signal Y showing that phase is lagging. A low-pass filter 6 controls oscillation frequency of a voltage control oscillator 7 in accordance with both the signals X, Y.