To provide a method for inspecting airtightness of high-pressure gas container and its device capable of performing more highly accurate inspection and implementing effecgive and a low-running cost inspection.

A mixed gas formed of gaseous helium and air is sealed and mixed under a predetermined pressure in a high-pressure gas container 100 and is set at a predetermined location in a chamber 2. After air is evacuated from the chamber 2, a sample gain in the chamber 2 is diffused for a predetermined time, the sample gas is extracted from the chamber 2, and a helium leak from the high-pressure gas container 100 is detected by a helium leak detector 12. At this time, the mixed gas is sealed under a pressure of 17 kg/cm2 G or higher, and the concentration of helium is set at 0.1-1.0%.