PURPOSE: To prevent the erroneous detection by receiving the signal wave-form caused by the electric discharge occurring in the fixed region and obtained by an antenna as one wave-form and counting the number of them to judge the defective vacuum.

CONSTITUTION: When the applied voltage V is a sine wave, the electric discharge occurs near the phase that the applied voltage V crosses zero and stops near the peak. The electric discharge occurring in this region (a) is received by an antenna as one wave-form of the signal wave-form (b), the number of the wave-forms is counted to judge the defective vacuum. If the vacuum is defective, the electric discharge occurs twice in one cycle, when a detecting circuit counts normally, 100 counts are obtained in one second for 50Hz, and 120 counts for 60Hz. Regarding the number of counts, some tolerance is given for the elapse time until specific counts are attained when the defective vacuum occurs, and the defective vacuum is judged if specific counts are attained in this range. Only the defective vacuum to be primarily detected can be thereby detected, and the erroneous operation by the external noise is eliminated.