PURPOSE: To make it possible to measure phase difference with high resolution and high accuracy.

CONSTITUTION: A waveform shaping circuit 2a shapes the waveform of a signal to be measured. A phase shifter 3 makes the phase of a reference signal variable according to the control voltage. This reference signal is inputted to another waveform shaping circuit 2b via the phase shifter 3. This waveform shaping circuit 2b shapes the waveform of this reference signal. A phase comparator 4 outputs and output signal conformed to the phase difference between those of output signals of both the waveform shaping circuits 2a and 2b. An amplifier 5 gives an output signal conformed to the output signal of the phase comparator 4 to the phase shifter 3 as the control voltage. Another amplifier 6 converts the output signal of the amplifier 5 into a specified standard voltage or standard current. The phase shifter 3, having a variable time constant circuit, alters time constant of this variable time constant circuit, thereby altering the phase of the reference signal.