PURPOSE: To give an automatic pressure control function automatically relieving pressure when the inside of a bag reaches specified internal pressure by interposing a thin-film between sealant layers and making the seal strength of the thin-film interposing section relatively smaller than that of a thin-film non-interposing section in one part of the bag.

CONSTITUTION: All of both side edges and bottom edges of surface and rear sheets 2, 2 are heat-sealed and heat seal sections 3 are formed in a bag 1. The sheets 2, 2 are composed of the laminates of base materials 2b and sealant layers 2a, and the inner surface sides are formed in the sealant layers 2a. A thin-film 4 is shaped in the cross direction at approximately the central position of the bag 1 under the state in which it is interposed between the sheets 2, 2. According to the constitution, the seal strength of thin-film interposing sections 3b is made relatively smaller than that of thin-film non-interposing sections 3a in the heat seal sections 3. Steam is generated by heating during cooking and internal pressure is increased gradually in a package in which the bag 1 is filled with food, but the seals of the thin-film interposing sections 3b are broken and internal pressure is made to escape when internal pressure reaches certain pressure, thus completely removing possibility in which the package is burst and contents are scatterred during cooling.