To solve the problem that airtightness of a fastener part is not maintained due to unexpected deflection, contraction and pulling force when an airtight fastener part of an inner bag body is slackened or wrinkled, while the inner bag body having an airtight fastener at an entrance is housed in an outer bag body, the area of the inner bag body is enlarged, it is pressurized and expanded, and the inner bag body is closely attached to the outer bag body in a slackened and wrinkled state conventionally.

The flexible inner bag body 10 having a finely air-permeable fastener 13 at the entrance of a user to a supplied high pressure gas is housed in an air-permeable and also pressure-resistant outer bag body 1. The high pressure gas is sent from a pressurizing source to the inner bag body 10, the outflow pressure of a gas is imparted over the entire finely air-permeable fastener 13 at the entrance of the inner bag body 10 in a pressurized and expanded state, and the part of the airtight fastener 13 of the inner bag body is prevented from being slackened or wrinkled.