To excellently execute bending of extruded material and at the same time to improve strength of the part of the extruded material subjected to bending.

A hollow extruded material 10 to be bent and a core 12 having plural ribs 14 positioned separately to each other in the longitudinal direction and two pieces of connecting parts 16, 18 to integrally connect these ribs are prepared. Then, the core 12 is inserted into the bending part of the extruded material, the core is located at a prescribed position by the use of two pieces of band shaped metallic sheets 20 with each end fixed respectively to both the ends of the core, and then in that situation the extruded material 10 is subjected to plastic working and is bent into desired shape. After bending is finished a comparatively strong tensile force is given to the band shaped metallic sheets 20 which are thereby removed off the core. The core 12 surely prevents the extrusion material 10 from generation of faults, and functions as a reinforcing member retained as it is after bending working.