To provide a cooking bag by a microwave oven which is reliably burst-opened by an increase in the internal pressure when the bag is heated in the microwave oven without using chemicals such as a release agent, and allows seasoning or the like to be spread to the entire food.

A plastic film to constitute a cooking bag 10 by a microwave oven which is opened by the increase in the internal pressure when the bag is heated in the microwave oven has a cut 40 which is not penetrated in the thickness direction. The cut forms a relatively weak portion against the tension by the internal pressure, the portion is reliably burst-opened by the increase in the internal pressure, and the seasoning 102 is spread to the entire food 101.