PURPOSE: To improve an operation considerably and to improve also the durability of a mold for a corner by fitting an auxiliary center core part to a thin plate part for a middle die so that it can be moved in the longitudinal direction of hollow extrusion molding parts which are inserted into a main center core.

CONSTITUTION: An auxiliary center core part 30 is fitted to a thin plate part 41 for a middle die so that it can be moved only in the longitudinal direction of a main center core 42. While hollow extrusion molding parts 1a and 1b are joined to the opposite ends(respectively, therefore, the auxiliary center core part 30 is displaced upward together with a main center core part 40 after a top force is removed, and a product is extracted from a cavity part 11 of a bottom tool 20 and the parts 1a and 1b on the occasion when the product is released from a mold after an L-shaped corner part 11 is die-molded. The product being pulled, then, a main center core part 40 is extracted from the product and separated therefrom and then the product is displaced and extracted from an auxiliary center core 31 for molding the corner part. During this operation, the auxiliary center core is held on the thin plate part 41 for the middle die by bolts 35, 35 and need not be supported by hand.