PURPOSE: To surely adjust the phase difference between first and second signals to a prescribed extent by preventing the second signal from being locked to the phase difference other than a desired phase difference from the first signal.

CONSTITUTION: A phase difference θ between the second signal which has the phase shifted by an infinite phase shifter 3 and the first, signal is detected by a phase comparator 7. A phase shifter control circuit 12 controls the extent of phase shift in the infinite phase shifter 3 based on the detected phase difference θ until the phase difference θ becomes a prescribed extent (for example, 0° or 180°). Voltage comparators 10 and 11 give such hysteresis to the control of the phase shifter control circuit 12 that the start of control of the extent of phase shift in the infinite phase shifter 3 which is performed in accordance with the increase of the phase difference θ by the phase shifter control circuit 12 is delayed at the time when the phase difference θ is the prescribed extent to stop the control of the extent of phase shift in the infinite phase shifter 3. A low frequency oscillator 13 releases this hysteresis to the control of the phase shifter control circuit 12 with a prescribed period.