To facilitate processing of spread paper by a cutting machine by reducing a developed length, and to preferably keep a degree of precision and strength at connecting corner parts between a bottom plate and an end plate.

The spread paper P1 of a coffin body 12 has side plates 12B beyond longitudinal V grooves y1 on both sides in the transverse direction of the bottom plate 12A and lower end plates 12C beyond transverse V grooves x1 on both sides in the longitudinal direction. Upper end plates 12D, 12D are provided beyond transverse V grooves x2 at positions where opening end parts of the side plates 12B are extended in the longitudinal direction. The longitudinal dimension of the upper end plates 12D, 12D is set at a length shorter than the height dimension of the coffin body 12. When the spread paper P1 is assembled, the side plates 12B and the lower end plates 12C are folded along the longitudinal V grooves y1 and the transverse V grooves x1 relative to the bottom plate 12A, and the upper end plates 12D, 12D are folded along the transverse V grooves x2 relative to the side plates 12B so that the lower end plates 12C and a pair of the upper end plates 12D, 12D are connected on the same plane.