PURPOSE: To form a flexible groove of a linear or curved line in a surface and express recessed/projecting various patterns on a plane material surface used as a folded box such as wood shavings by cutting the surface of the plane material having styrene foam used as a substrate.

CONSTITUTION: A cutting edge 3 of a cutting part 2 is projected from the bottom to a carrier line 1 surface for a styrene foam plate 6 and the surface (bottom face) of the styrene foam plate 6 under being carried is cut by the cutting edge 3. In this case, if the cutting edge 3 is fixed, a linear recessed groove 4 is formed in the surface and, if the cutting edge 3 is moved perpendicularly to the styrene foam plate 6, a curved recessed groove is formed in this plate 6. The styrene foam plate 6 having various recessed/projecting patterns formed in its surface is used as a folded box material such as wood shavings.