To eliminate such a phenomenon that a metal layer electrified by application of microwaves maintains an electrified state to cause sparks by cutting four corners of a bag in bagged food wherein a composite film with the metal layer laminated at an intermediate part is heat-sealed into the bag with food filled inside.

A composite film F for heat-sealing comprises laminate of, from a surface, an insulation layer 1 such as a nylon film which is to be a function layer, a metal layer 2 of an aluminum foil to be a microwave shielding layer and a heat-sealing layer 3 such as a polyethylene film. The composite film F is heat-sealed H into a bag with food filled inside. Then four corners C...C of the bag P are cut to remove sharp parts. As a result, microwaves are prevented from reaching the food inside, and also bagged food free from spark generation can be obtained. In addition, this can be utilized for a plurality of kinds of cooked food to prevent a particular kind of food from being heated.