PURPOSE: To enable the extrusion molding of a wide material using a small die in size by controlling the metal flow of an extrusion which is passed through the curved molding slit of the die.

CONSTITUTION: The die 2 is the one whose molding slit 3 is formed into a circular-arcuate molding slit 3 corresponding to the shape of a cross section in the state where the wide plate material 1 is curved into a circular-arcuate shape of a prescribed radius of curvature in the width direction. The metal flow of the material E1 to be extruded which is passed through the circular- arcuate molding slit is controlled by a metal flow controlling means 4. The metal flow controlling means 4 is composed as the one in which the bearing lengths LD, LU of upper and lower bearing parts 6, 7 opposed to each other of the molding slit 3 are made to differed. In this way, the curved molding slit whose circumscribed circle is made small by making into the curved shape is used for the die 2.