PURPOSE: To easily form an opening by providing a front cut and a rear cut on a wall face of a bag body at a certain distance in a direction perpendicular to a stretch direction of a uniaxial stretched film and by sticking a lid material over the entire cuts.

CONSTITUTION: When a lid material 6 is peeled off a bag body 1 with a lid material tab 7 of the lid material 6 held, pressure-sensitive adhesive agent applied to the lid material 6 allows an inner part of a front cut 4 provided on a wall face 2 to be peeled off adhering to the lid material 6. At the same time, a laminate body for constituting the bag body is torn at both ends of the front cut 4, so that an upper wall 2 of the bag body 1 is torn along a tearing line 8 parallel to a stretch direction of a uniaxially stretched film. When a tear along the tearing line 8 reaches a portion of a rear cut 5, it is stopped there so that the tear does not processed further. If the lid material 6 is further peeled off, only the lid material 6 is peeled off from the wall face 2 at the point when the rear cut 5 is passed. At this time a part enclosed by the front cut 4 and the rear cut 5 on the side wall 2 and the tearing line 8 is peeled off adhering to the lid material 6.