To form, in a bag, an auxiliary storage portion that facilitates storage of a comparatively large load or the like unstorable in the main storage portion and that is unobstructive when not in use.

The method of forming the auxiliary storage portion is such that to a mounting face 2 for attaching the auxiliary storage portion in the bag, a lower part of a front sheet 3 is attached which serves as the front plate of the auxiliary storage portion, and that a pair of gusset pieces 4 are attached in one side to the mounting face 2 with a space apart in a manner making the mutually opposing side be a free end so that in the opposite mounting face side of the front sheet 3, each gusset may be made freely changeable from a collapsed state to a raised state toward the side opposing each other, and in the method, the pair of gusset pieces 4 are communicating with string-shaped communicative means 6. Thus, the bag 21 includes the main storage portion 22 and the auxiliary storage portion 1, and the auxiliary storage portion 1 is formed by the method.