A steel strip is fed and compressed between a driving roller and a compression roller to generate a stress greater than a yield stress in the steel strip and to elongate one periphery portion of the steel strip, which is on one side, more than the other periphery portion of the steel strip, which is on the other side, in a sending direction. The compression roller includes a first contact portion and a second contact portion. The second contact portion extends from an end of the first contact portion in the axial direction of the compression roller. The end of the first contact portion has an outer diameter less than an outer diameter of the second contact portion.