PURPOSE: To easily design a circuit for measuring phase differences by measuring the phase difference between signals output from first and second power lines via first and second input circuits, and finding the phase difference between both lines according to the difference between the results of measurement.

CONSTITUTION: The common terminals 5c, 6c of changeover switches 5, 6 are connected to one set of respective terminals 5a, 6b. A first power system side voltage signal is output from a first input circuit 2 to a phase comparing circuit 4 and also a second power system side voltage signal is output from a second input circuit 3 to the circuit 4. At the circuit 4 the phase difference between both of the signals output from the respective circuits 2, 3 is measured. When common terminals 5c, 6c of the switches 5, 6 are connected to the other set of respective terminals 5b, 6a a first power system side voltage signal is output from the circuit 2 to the circuit 4 and also a second power system side voltage signal is output from the circuit 3 to the circuit 4. At the circuit 4 the phase difference between both of the power systems is found according to the difference between both of the phase differences.