The backpack encompasses a box-shaped storage-compartment having first and second main surfaces opposite to each other and a triple-sided surface whereby triple continuous sides of the first main surface and triple sides of the second main surface opposite thereto are connected to each other, and a back plate encompassing a joint plane opposite to the first main surface and a backrest plane opposite to the joint plane. A convex element is disposed on one of the first main surface or the joint plane, and an insertion hole with a structure matching the convex element is disposed on a position on the other of the first main surface or the joint plane corresponding to the convex element. The storage-compartment and the back plate can be freely attached to and detached from each other by placing the convex element into the insertion hole and separating the convex element from the insertion hole.