PURPOSE: To easily form a plate-like shape which has recessed/-projection lines on the surface of one side into a flat shape and also improve the yield of product by cutting dummy recessed/projection lines into the flat shape after extruding the plate-like shape which has the surface having the recessed/projection lines and the dummy recessed/projection lines on the opposite surface.

CONSTITUTION: After extruding the plate-like shape 20 which has the surface having the recessed/projection lines 13 and the dummy recessed/projection lines 14 on the opposite surface, by cutting the dummy recessed/projection lines 14 into the flat shape, the plate-like shape having the recessed/projection lines 14 on the surface of one side is formed into the flat shape. And, by equalizing the surface area of the recessed/projection lines 13 to the surface area of the dummy recessed/projection lines 14, the frictional resistance of metal and flow velocity of metal or the like on the sides of the recessed/projection lines 13 and dummy recessed/projection lines 14 at the bearing of die for extrusion at the time of extruding the plate-like shape are equalized. In this way, the extruded shape having high-accuracy flatness is obtained and cutting accuracy after that is improved.