PURPOSE: To measure the phase difference without using a special element by logically discriminating whether the phase difference between two pulses is within a prescribed range or not.

CONSTITUTION: Pulse signals (a) and (b) are inputted to clock inputs CK of flip flops (FF) 11 and 12 respectively, and the FF 11 is reset before the FF 12, and respective outputs Q are changed with the same time difference. When both outputs Q go to the high level together, an AND gate 13 outputs the signals in the high level to both reset inputs R to reset FFs 11 and 12. Therefore, a pulse signal having the same time width as the phase difference between two pulse signals appears as a signal (c), and a whisker pulse signal due to signal delay between elements appears as a signal (d). Signals (c) and (d) and signals obtained by delaying them by a prescribed time are inputted to logic circuits 4 and 5, and signals (e) to (j) are outputted after logical operation. Thus, it is logically discriminated whether the phase difference between two pulses is within the prescribed range or not.