PURPOSE: To prevent the inner peripheral surface of an obtained extruded hollow-shaped aluminum shapes from being flawed.

CONSTITUTION: Before a male die 1, female die 2 and die ring 3 into which both these dies 1, 2 are fit are combined, they are preheated in the air. After completion of preheating, the layer of oxide is removed by grinding the bearing surface 1a of the male die 1. Next, both dies 1, 2 and the die ring 3 are combined and mounted on an extrusion device and a material is extruding formed with this extrusion device.