To make the forming of a flange having a prescribed shape possible at low energy when forming the flange by expanding the end part of a tubular material made of aluminum by electromagnetic forming.

The flange 22a is formed by expanding the end part of the tubular material 14 made of aluminum to a prescribed angle (the half angle ψ of inclination) by preliminarily performing press forming with a press die 20, inserting an approximately truncated cone-shaped coil 21 for electromagnetically forming into the end part and further expanding the expanded end part by electromagnetic forming. Energy required for electromagnetic forming is reduced by the amount equivalent to the preliminary work to the half angle ψ by press forming. The coil 21 can be arranged near by the end part of the tubular material by making the half angle α of inclination of the coil 21 approximately equal to the half angle ψ of inclination of the end part of the tubular material.