PURPOSE: To prevent a malfunction of the transformer in starting by applying a suppression with the direct current and the second harmonic components when started and changing it to a suppression of only the second harmonic component after a fixed time.

CONSTITUTION: When the transformer is started, both circuits of the direct current component filter 11 and the harmonic component filter 12 are formed, the direct current and harmonic components which are contained in the exciting rush current are made suppressing outputs through the respective filters to prevent a malfunction of the differential relay. An output of the direct current component filter 11 is hindered by the voltage detecting element 13 of the primary side of transformer after a fixed time from the starting of transformer. Therefore, even though a direct current component is contained in the difference current when a trouble takes place in the differential circuit, the differential relay is exactly operated without an influence of the direct current component since the output of direct current component filter 11 is obstructed.