To enable an effective air vent, and to enable a manufacture of a package showing high sealability and excellent convenience in easy transportation, exhibition, or the like.

The air vent device includes a pressure 41 capable of stopping a tubular film 25 packaging a packaged object 14 from above, and a moving mechanism liftably moving the press member. The pressure has a sealed bag body 43 and a base plate 42 to which the bag body is attached. An amount of filled gas in the bag body is such that the gas is freely movable in the bag body, and that the lower surface of the bag body can meet the shape of the packaged object via the tubular film. This narrows the space of the tubular film placed between the bag body and the packaged object, and then the inside air is sent backward.