PURPOSE: To enable the measuring of a phase difference accurately, handily and directly along with a cost reduction and a simplified measuring work, by using a digital data indicating instantaneous values of two AC quantities of electricity to perform a computation and a processing.

CONSTITUTION: Several instantaneous values of two AC quantities of electricity different in the phase with the same frequency are sampled to obtain a digital data by an A/D conversion 1. Then, a sine value and a cosine value of the phase difference are calculated 2 by applying a addition system the computation theory in a digital relay. Then, assumed phase angles corresponding to the sine value and cosine value are calculated 3 from a sine value table. Thereafter, the assumed phase angle is converted 4 to a true phase difference by a specified conversion formula according to an attribute quadrant of the true phase difference judged from positive or negative of the sine value and the cosine value and the resulting phase difference in displayed 5 digitally.