To provide an assembly octagonal casket made of cardboard which structurally keeps a sufficient strength after the assembly not to add a reinforcing member afterward and creates an exclusive atmosphere with piquancy by making the casket into a three-dimensional octagon.

A main body A of the casket is formed into an octagonal box shape which comprises an octagonal bottom plate part 1 having slanting sides at four corners of a rectangle, side plate parts 2 rising on both sides, end plate parts 19 rising on the front and back sides, and chamfering plate parts 4 rising on the slanting sides and has an opened upper surface. Inward folded plate parts are superposed on respective upper sides of the side plate parts, the end plate parts 19, and the chamfering plate parts to integrate. A lid body B is formed into an octagonal, shallow box shape which has the bottom up and an opened lower surface and comprises a rectangular top plate part 15, slanting plate parts 16, triangular slanting plate parts 17 between the slanting plate parts 16, the side plate parts 2, the end plate parts 3, and the chamfering plate parts 4. The inward folded plate parts 23 are superposed on respective lower sides of the side plate parts, the end plate parts 3, and the chamfering plate parts 4 to integrate.