To prevent equipment actuated by the output of a current differential relay from malfunctioning when the level of higher harmonic components of differential current in the current differential relay is lower than the actual level.

A current differential relay, which outputs actuating output TS when the differential current is equal to or above a prescribed level and the ratio between the input signal and the differential current before the difference is taken is within a prescribed range, is provided with locking means 8A, 8B, 8C, 8D that output locking signals to the actuating output when the content ratio of the second higher harmonic component in phase currents YIa, YIb, and YIc on the Y side is equal to or above a prescribed level.