PURPOSE: To form the round corner of a box, in high productivity, by cutting a groove to the reverse side of a PVC-laminated plywood in such a shape as to form a cylindrical form when the plywood is bent at the part of the groove, applying a flexible tape across the groove, bending the plywood, and filling the cylindrical groove with a foamable resin.

CONSTITUTION: A PVC-laminated plywood composed of a PVC sheet 6 and a plywood board 7 is provided with a groove 8 having a shape to form a cylindrical shape when the board 7 is bent to a desired angle. A flexible tape 14 is bonded to the board 7 across the groove 8. The board 7 is bent at the groove 8 to the right angle, and a foamable resin is poured into the space 15 formed with the tape 14, the PVC sheet 6 and the board 7 to form the desired round-cornered box.