PURPOSE: To obtain a product having a mirror-like smooth inner surface without causing a seizing, by using a punch having the surface on which a carbide coated layer is formed, in a process of back-piercing extrusion to form a product having a hollow cross section from a metallic cylindrical blank material.

CONSTITUTION: A blank material 1 is inserted into a die hole 2 formed with a die 4 and a knockout 10, and then, a carbide coated layer 9 is formed on the surface of the material 1, and a punch 3 having an expanding part at its tip is inserted to work the blank material 1 by a back-piercing extrusion. After that, a formed extruded piece 6 is taken out from the die 4 together with the punch 3 in one body, and at the stage of taking out these things from the hole 2, the piece 6 contacts with a stripper 11 to be removed from the punch 3. Hereupon, the contacting surface between the punch 3 and the material 1 is allowed to be in a non-lubricant state, and the surface of layer 9 is desirable to be made as smooth as a mirror.