PURPOSE: To make it possible to place a cover without leaving air in a container when the mouth of the container is sealed with a cover by heat-welding.

CONSTITUTION: A filling nozzle 9 for a content and an inert gas is inserted from a mouth part 5a of a container 5, and the content and the inert gas are introduced into the container 5. Then, the upper surface of the mouth part 5a of the container 5 and the lower surface of a cover 21 which is fitted on the mouth part and is heat-weldable to the mouth part of the container are heat-molten. At the same time, the inert gas is blown in the mouth part of the container, or in the mouth part of the container and the internal space of the cover 21, through a nozzle 27, and the container mouth part 5a and the cover 21 are brought into press-contact, and the container mouth part 5a is welded and sealed to the cover 21.