PURPOSE: To avoid noise influences to enhance the reliability of a system and make the system compact, by setting an optical magnetic sensor in a line, converting an output from the optical magnetic sensor to an electric signal and detecting whether the line is charged or shut down.

CONSTITUTION: Optical magnetic sensors 2A-2C are set in a power feed line 1, and outputs of the sensors are guided to O/E converters 4A-4C in a digital operating device 8 via optical fibers 3A-3C and converted to electric signals. The electric signals are compared with a predetermined value at comparators 5A-5C, and converted to 'high' or 'low' signals. A logic circuit 6 operates 2outof3(3C2) based on the outputs from the comparators 5A-5C. For example, if two or more of the three outputs are 'high', it is detected that a voltage is present. If two or more of the three are 'low', it is detected that the power line is shut down. A failure detection circuit 7 detects from outputs of the O/E converters 4A-4C whether or not a system is abnormal. When the abnormality is detected, which of the system fails is informed.