PURPOSE: To enhance the sewing strength of the lower edge of a bag by a method wherein each bag blank of flat cylindrical cloth sheet is provided at the lower edge cut off therefrom with triangular and arcuate cuts alternately and the edges are temporarily sewn together, folded back and then permanently sewn together in a flat form.

CONSTITUTION: A flat cylindrical cloth sheet is cut into large blanks U of a predetermined length. From each blank U are removed the upper corner parts (a) and (b), the lower corner parts (c) and (d), the lower central part (e) and intermediate parts (f) and (g) between the parts (d) and (e), all the aforesaid parts being shown by shade. An inserting opening part 1 is formed at the edge of removed cut parts (a) and (b) and takeout opening parts 3 are formed at the edges of removed cut parts (f) and (g) and the blank is thereafter folded double along the center line 2. The edges of the removed lower corner parts (c) and (d) are sewn along a temporary sewing line 5 to form a temporarily sewn line 5' and folded back along the temporarily sewn line 5' and, along the resulting sewing line 5", two securing flat sewings 6 are made to form the lower lid of the bag.