PURPOSE: To make it possible to measure a phase difference in the display range of 0 to 360° by executing computation on the basis of the phase difference determined by a digital signal processor (DSP) and a leading or lagging phase signal obtained by a lead/lag detecting circuit.

CONSTITUTION: A voltage signal (v) and a current signal (i) are taken in DSP 3 through input circuits 11 and 12 and A/D converters 21 and 22 respectively and a phase difference between the signals (v) and (i) is determined. Meanwhile, the signals (v) and (i) are given to zero-cross comparators 61 and 62 through the input circuits 11 and 12 and a zero-cross point thereof is detected. Outputs of the comparators 61 and 62 are added up by a lead/lag detecting circuit 7 and a leading or lagging phase of the signals (v) and (i) is detected, and a detection output thereof is given to CPU 4. The CPU 4 executes computation on the basis of the phase difference between the signals (v) and (i) determined by the DSP 3 and a signal of the leading or lagging phase detected by the detecting circuit 7 and determines a phase angle. This phase angle is displayed in the range of 0 to 360° in a display unit 5 under the control of the CPU 4.