A backpack including a first backpack member provided on both sides thereof with shoulder straps, a second backpack member configured onto a rear surface of the first backpack member, and an auxiliary loading portion 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, the backpack including a buckle for coupling at least both side portions of the first backpack member and the second backpack member to each other, wherein the auxiliary loading portion is formed by coupling a pair of panels to each other by a first hinge so as to be folded inward only, in which one of the pair of panels coupled by the first hinge is rotatably fixed to the first backpack member via a second hinge therebetween, and an opposite one of the pair of panels coupled by the first hinge is rotatably fixed to the second backpack member via a third hinge therebetween, so that a gap between the first backpack member and the second backpack member is prevented from being widened, and the first hinge and the pair of panels folded only inward prevent an article loaded on the auxiliary loading portion from sagging, thereby preventing a center of gravity of the backpack from being changed.