A packaging bag composed of a multilayer film and having a steam releasing mechanism. The multilayer film has at least a base layer and a welding layer and has a moisture content of not more than 1.0% in a state in which the packaging bag is filled with a content. The steam releasing mechanism has at least a steam releasing portion formed to have a maximal width of 3 to 20 mm. The internal pressure of the packaging bag is kept to be not more than 115 kPa during a period from the time the internal pressure is released through the steam releasing portion as a result of an increase in the internal pressure caused by heating in a microwave oven to the time the heating in the microwave oven finishes.