PURPOSE: To properly recognize the state of external information even if the control power supply of a power system protection controller is momentarily interrupted when inputting external information such as the switching of a breaker with a contact signal and recognizing the state.

CONSTITUTION: The normally open contact signal and the normally closed contact signal of an auxiliary relay 9 for detecting the control power interruption of a device are inputted via an input circuit 4 and the contact signal of external information is not updated when the control power supply is interrupted and a previous value is retained. Also, with the external information, the normally contact signal and the normally closed contact signal are inputted, the logic conditions of both contact signals are established, and a previous value is retained when the control power supply is interrupted.