An end of a stay material made of a tubular aluminum alloy extruded material is expanded by electromagnetic molding to form a flange for producing a bumper stay. The flange width is increased without cracking and a decrease in the thickness. A stay material having a circumferential wall undulating in a waveform pattern is used. A circumferential length L (one round length along waveform pattern) of the outer periphery wall along the circumferential direction of the stay material is longer than a circumferential length L0 of a simple cylindrical circumferential wall having the same outer diameter (circumscribed circle) (L>L0). L−L0 is an excessive line length. Since the circumferential wall has the excessive line length, when the end of the circumferential wall is expanded to form the flange, the flange having a large diameter can be formed.