To form a mounting flange 27 at the end of a tube 21 composed of an aluminum alloy extruded material, and also to make mounting holes 28 in the mounting flange 27.

A tube 21 is surrounded with an electromagnetic forming die 22 having a through hole 24, with the end of the tube made to be projected out from the end face 23 of the die 22 for a prescribed length, and with an electromagnetic forming coil body 26 inserted. The peripheral wall at the end of the tube is expanded radially outward by electromagnetic forming and pressed against the end face 23 to form a mounting flange 27 at the axial end. A plurality of projections 25 with a pointed tip end are installed at the end face 23 of the die 22, so that the peripheral wall at the end of the tube expanded at the time of electromagnetic forming comes into contact with the projections 25. The pointed tip end of the projections 25 breaks through the thickness of the peripheral wall at the end to form a holes 28 therein. The hole 28 is made to serve as a prepared hole for a tapping bolt or used as a positioning mark for punching a bolt hole.