PURPOSE: To improve the yield of product by forming the bearing of a die used for extruding a plate-like shape into a circular-arc shape the whole of which is projected toward the side of recessed/projection lines and passing metal through the die.

CONSTITUTION: The bearing 11 provided on the die 10 for extrusion consists of a flat circular-arc-shaped base hole 12 which is gently projected toward the surface of one side and the recessed /projection lines 13 which are continuously ranged on the side on the projecting side of the base hole 12. The recessed/projection lines 13 are in a shape in which calking parts 5 and fixing wall parts 6 for clamping fins together with the calking parts 5 are alternately provided between groove lines 3. The curvature of the bearing 11 is set taking the shape, dimensions, material, etc., of the shape 20 to be molded into consideration. The die 10 for extrusion is fixed on the front side of a container 30 and, when heated metal 31 is pressurized in the direction of the die 10 with a stem 32, the metal 31 is passed through the bearing 11 of the die 10 and formed into a roughly comb-like shape in the cross section. At this time, since the area of the recessed/projection lines 13 is larger than that of the flat surface, the flow velocity of the metal becomes slower and the shape is formed, into the flat shape.