PURPOSE: To generate an alarm by a buzzer and to acutomatically open a switch apparatus when vacuum deterioration is generated, by attaching a vacuum sensor to a vacuum switch tube while always monitoring the vacuum degree of the vacuum switch tube.

CONSTITUTION: A commercial power source is connected to an input terminal 12 and an optical signal control circuit 14 is energized to irradiate a vacuum sensor 16 with light. The sensor 16 is distorted corresponding to the vacuum degree of a vacuum switch tube 10 and receives reflected light corresponding to the vacuum degree. The reference voltage from an input control circuit 13 and the voltage from the optical signal control circuit 14 are compared in a detection circuit 17 and, when the voltage from the optical signal control circuit 14 becomes lower than the reference voltage from the input control circuit 13, a first and a second transistors 18, 19 are brought to continuity. Therefore, an alarm apparatus 22 is energized to inform the lowering in the vacuum degree of a vacuum switch tube 10 and a switch apparatus 9 can be separated from a superior circuit.