PURPOSE: To prevent peeling of a sealed part in thermal sterilization, by placing a food into a bag having a cut window, provided near an opening part and closed with a sterilizing filter, sealing the opening part, then sterilizing the food and sealing the part under the cut window.

CONSTITUTION: A cut window 3 closed with a sterilizing filter 4 capable of passing air and steam but no microorganism therethrough is provided near an opening part 2 of a packaging bag 1, e.g. polyethylene, polypropylene, etc. Foods, e.g. vegetable, meat, noodles, etc., are contained from the opening part 2 in a part under a line (b) below the cut window 3 and the opening part 2 is sealed at a part (a). The sealed bag is then thermally sterilized, cooled and then sealed at the part (b) below the cut window 3 to prevent oxidation of contents. Sealing is preferably carried out by sealing under reduced pressure and the upper part, if necessary, is cut to afford the aimed packaged food.