To obtain a packaging box that can be heated even with a household microwave oven by a method wherein a severable double perforation is provided along scores extending in parallel with one side of the box to the top, to both sides, and to the base.

A flat box is made with its top and base being made in the same square, and a circular sheet of microwave heating element is softly stuck to the inside at the base of the box. Two perforations 1, being in parallel with each other, are provided, being in parallel with one side of the box, at positions separating respectively by 2cm and 6cm from the side. On heating a pizza, the box is severed at the perforation 1 separating by 6cm from the side. Then the box, with the pizza being put therein, is placed upside down, namely with the base to which the circular sheet of microwave heating element is stuck placed to face upward, on a turn table of a household microwave oven in a normal size, and the pizza is heated together with the box. As the pizza is held under an appropriate protection by the top of the box while being heated, the pizza itself can be prevented from being heated excessively.