To eliminate deformation after cooling, buffer a filling temperature, reduce filling time and facilitate inspection for fine leakage with applied pressure held by coating a cap with a cylindrical body while a plug is slightly screwed into the cap, applying pressure to inside a drum with gas led and further tightening the plug for sealing.

Since airtightness of a plug (c) to be used for applying pressure and sealing a drum depends on a degree of tightening of the plug (c), if a cap- like cylinder 1 is put over with the plug (c) slightly screwed initially and pressurized gas is blown in from an intake port 1, air is fed through a slit between the plug (c) and a cap screw (b) to increase a pressure inside the drum. Subsequently, after the plug (c) is further tightened by a plug rotating means such as a spindle 2 in a pressurized state, this apparatus is removed. Thus inefficient operation at the time of filling can be eliminated, shrink deformation of the drum at the time of cooling after high-temperature filling can be prevented, and leakage inspection on a fine region in leakage inspection can be easily performed.