PURPOSE: To prevent generation of product flaws by setting the outlet side roll face angle of main rolls to be larger than a specific angle, forming a small projection on the top of a plug, and setting a side peripheral face taper angle of the projection to be smaller than a taper angle of the plug body itself.

CONSTITUTION: A roll face angle α2 of an expansion part 13 of main rolls 11, 11 is set to a value of ≥5° and a small diameter part 19 is formed on the top part of a plug 14. The shape of the top of the part 19 is sphere and the side peripheral face of the part 19 is formed of a tapered face having a taper angle smaller than a taper face angle 2θ2 of the body part of the plug 14. When a rolled stock 5 heated to a prescribed temp. is bitten by the main rolls 11, 11, the stock 5 abuts on the top of the projection 19. In that time, a distance (d) where the stock 5 moves for abutting is shortened and an acting region of rotation forging effect for the stock 5 is narrowed. Thus, generation of product flaws caused by forging cracks in an expansion rolling part by the main rolls 11 and the plug 14 in rolling having an expansion ratio of ≥1.1.