To satisfactorily perform the positioning of a structure to a machining part when an upright plate is positioned in its outer surface and mounted on the structure by half-pressing an upright plate of extruded material to a minimum thickness or less in the tolerance range of thickness, and then machining the bottom plate on the basis of the inner surface of the upright plate.

In the machining of an extruded material 1, an upright plate 1b is half-pressed so as to have a thickness (x). It is then pressed by use of a metal mold having the inner surface of the upright plate 1b as a reference surface to work a hole 1d. The upright plate 1b is fastened to a structure 2 by a screw 4 inserted through a hole provided therein. The part making contact with the structure 2 forms a machining surface 1c. A shaft 2a provided on the structure is inserted through the hole 1d. Since the tolerance of thickness of the upright plate 1b is 0, the positional relation between the hole 1d and the shaft 2a can be made concentric.