Detection of degradation of the degree of vacuum in a vacuum interrupter suffers from poor sensibility because of mixed presence of various noises other than that due to discharge resulting from degradation The present invention allows accurate detection of degradation of the degree of vacuum by detecting the continuity of discharge produced between an electrode and a shield due to degradation of the degree of vacuum and the duration of discharge. The continuity of discharge is detected in accordance with a time slightly longer than 1 cycle time of a power-source voltage, and the duration of discharge is detected in accordance with a time sufficiently longer than 1 cycle time.