Contour of a storage section of a three-sided bag is formed of (1) two side inner lines in a longitudinal direction, (2) two tilt inner lines which tilt inside with forming an obtuse angle with each side inner line from a lower end of each side inner line, and (3) one lower side inner line in a lateral direction which connects lower ends of respective tilt inner lines. A longitudinal size “a” of an intersection IPa of the side inner line and tilt inner line on the basis of an intersection BP of a side outer line or its extension, and a lower side outer line or its extension is preferably within a range of 0.08A to 0.71A, and a lateral size “b” of an intersection IPb of the tilt inner line and lower side inner line on the basis of an intersection BP is preferably within a range of 0.12B to 0.48B.