To obtain the state of sterilization that is much more satisfactory with decline of temperature at the surface of contents prevented by a method wherein a deaeration part is provided at a weld part on a bag mouth, and an incomplete weld part is made weaker in welding strength than vertical weld parts and the weld part on the bag mouth and separable in accordance with rise of internal pressure.

The number of deaeration part 3 may be one, or two or more for one weld part 2 on a bag mouth. An incomplete weld part 6 is provided at the inside of the bag mouth weld part 2. The incomplete weld part 6 must be made with the bag mouth welded airtightly and there should be no omission in welding. Although the incomplete weld part 6 is virtually welded completely, it is weak in welding strength and must be separated easily when internal pressure rises with a sealed bag heated by microwave heating. Namely, welding in such case must not be such that a film, being a bag material, is torn up. As heating progresses and internal pressure in the bag exceeds a specified level, the incomplete weld part 6, weaker in welding strength as compared with other parts, separates, and water vapor is discharged through the deaeration parts 3 provided at the bag mouth weld part 2.