To prevent an opening piece from being accidentally pulled to improve reliability in packaging by forming one of films at an opening part shorter than the other film and forming the opening piece with a cut provided on the shorter film.

Two plastic films 21, 22 having the same width are supplied while slightly shifted in a widthwise direction, and they are fuse-sealed in a trapezoidal form to obtain a bag with sides 4a, 4b sealed. Then an upper seal 5 is heat-welded to form a housing 6 and an opening part 7. In this case, one of the films 22 is formed with the opening part longer and with an opening shorter, while the other film 21 is formed with the opening part shorter and with the opening longer. Further, a base plate or the like made of rubber is put between the films 21, 22 at the opening part 7, and cuts 8a, 8b are provided on the opening part of one of the films 21 by a cutter blade to form an opening piece 9, thereby completing a packaging bag. Thus an end of the opening piece is protected, so that the bag housing may not likely to be torn due to the opening piece 9 accidentally pulled.