PURPOSE: To conduct efficiently an airtightness test at high accuracy for a short time by sealing plural electrodes in an airtight sealing part and monitoring a grow discharging waveform when a high voltage is impressed between the electrodes.

CONSTITUTION: A pair of electrodes A and B are sealed in an inside part 2b sealed by the airtight sealing part 2a. The high voltage is impressed between pulling-out terminals A' and B' introduced from a pair of electrodes A and B. How to impress the high voltage is that an inductive load L is connected between the terminals A' and B' and the high voltage arising when a switch K is disconnected is utilized. If the airtightness at this time is defective, the grow discharging waveform between the electrodes A and B develops disturbance: otherwise, said waveform becomes normal. Accordingly the grow discharging waveform is monitored, whereby such decision is made whether the airtightness is good or not.