To properly heat-cook foodstuffs sealed with a packaging material as it is by means of a microwave oven.

A tray 2 and a cover material 3 are heat-sealed to package foodstuffs 6 airtightly. Projecting ends 4a, 4b are provided at a corner of the tray 2 and cover material 3 to interpose a heating unit 5 therebetween. If the foodstuffs are cooked with its packaging material tightly sealed by a microwave oven, the heating unit 5 absorbs microwaves to generate heat. Until the sealing strength of a sealant layer 7 falls sufficiently by the generated heat, a proper heat cooking is performed in water vapor generated within the packaging material. If the temperature of the unit 5 reaches a predetermined temperature and hence the sealing strength of a heat-sealed part falls, the pressure in the packaging material is exerted on the heat-sealed part so that the heat-sealed part around the unit 5 is unsealed to allow the water vapor within the packaging material to escape to the outside.