PURPOSE: To provide a highly reliable phase measuring apparatus which operates definitely.

CONSTITUTION: The phase difference measuring apparatus comprises a circuit 24 for detecting the phase difference between first and second input signals confined within a predetermined indefinite region, circuits 20, 22, 30 for shifting the phase of any one of the first or second input signal by a predetermined angle upon detection of the phase difference between first and second input signals confined within a predetermined indefinite region, and a circuit 10 for detecting the phase difference between one signal subjected to phase shift and the other signal not subjected to phase shift. Phase difference between first and second input signals is determined based on the detection results of the phase difference detection circuit and the predetermined phase angle.