PURPOSE: To detect the gas leakage from the flange contact surface by providing a plurality of O rings concentrically onto the flange surface of a gas-charged container and by measuring the pressure between these O rings.

CONSTITUTION: A flange 2b of a blind cap 3 is connected to a flange 2a of a compressed-gas-sealed airtight container in which the gas with pressure higher than the atmospheric air is charged. In order to keep a pair of flanges 2a and 2b airtight, an O-ring 4a for main gas sealing and an O-ring 4b on its outside for backing up are provided to the concentric grooves of one of the flanges, i.e. 2a. A gas detecting hole 2d is provided to a flange contact surface 2c between these O rings 4a and 4b and connected to a pressure gauge 7 with a gas piping 5 and a valve 6.