PURPOSE: To always ensure a correct measurement of the phase difference regardless of the phase fluctuation of the input signal, by counting the clock pulses with a presetable counter in accordance with the phase difference.

CONSTITUTION: The phase difference deciding circuit 6 produces the phase shift output when the phase difference between the input signal B and the reference signal A or the shifted 5 reference signal A' is smaller than the prescribed value. According to this output, the phase shift circuit 5 gives a phase shift or no phase shift to the signal A. At the same time, the preset circuit 30c presets the value corresponding to the fixed amount of shift and then gives it to the counter 30a. On the other hand, the phase difference between the signals A' and B is measured 1 to form the gate pulse, and the clock C is applied to the counter 30a by the gate pulse and via the gate 20. Accordingly, the counter 30a counts the clocks according to the phase difference, and the contents of the count is displayed on the display unit 4.