To prevent buckling and deformation of bellows, i.e., an inner component, when gas is sealed in an inner part of a pressure container, and to enhance quality and durability.

This gas sealing method supplies gas from a gas supply port 10 into one of the chambers 8 of a pressure container 2 partitioned by a bellows 6, until a predetermined pressure is reached, and closes the gas supply port 10 after the supply of gas. In this case, as the pressure in the one chamber 8 is increased by the supply of gas into the one chamber 8, a pressure medium such as liquid or gas is supplied into the other chamber 9 to increase pressure within the other chamber 9, thus supplying gas into the one chamber 8 while keeping the differential pressure between both chambers 8, 9 at a magnitude that is equal to or lower than a predetermined value.