PURPOSE: To sterilize a packed food under ordinary pressure, by filling a food in a bag having partially a vent part, primarily heating the bag in a support transmitting microwaves to seal the bag, and secondarily heating the bag by irradiation with microwaves while reversing.

CONSTITUTION: A food with ≥20% moisture content is filled in a food packing material, transmitting microwaves, and having the inner surface with heat sealability to leave a vent part, and the packing material is then contained in a support made of a material transmitting microwaves, and in the obliquely placed state transferred to the first heating 11 by a conveyor. The packing material is then reversed while heated to 100W105°C by microwaves, pressurized high-temperature steam or both together, passed through a holding part 12 and deaerated. The vent part of the resultant packed food is sealed up, reversed while heated to ≥125°C by irradiation with microwaves in the second heating part 13, passed through a holding part 14 and cooled in a cooling part 15.