The present invention provides a backpack structure for easily loading a large item having column shape or “T”-shape that cannot be put into a storage space of a backpack. The backpack (500) includes a first backpack member (510) provided on both sides thereof with shoulder straps 530, a second backpack member (520) configured onto a rear surface of the first backpack member, an auxiliary loading portion (540) configured to fix the bottom surfaces of the first backpack member and the second backpack member to each other to enable the first backpack member and the second backpack member to be unfolded in opposite directions, and configured to be exposed outward from the first backpack member and the second backpack member, and a buckle (550) for coupling both side portions of the first backpack member (510) and the second backpack member (520) to each other, wherein the auxiliary loading portion 540 includes a spacing adjusting device capable of adjusting a spacing according to the item loaded between the first backpack member and the second backpack member.