The present invention includes a bar having a maximum cross section region smaller than a minimum cross section region of a hollow portion of a pipe P, and projected portions mounted along an axial direction of the bar and including a projected portion with maximum extent Hb, extent Ha of the projected portion positioned at a leading end side of the bar is set to be smaller than extent Hb of the projected portion positioned at a center side in the axial direction, and extent Hc of the projected portion positioned at a base end side of the bar is set to be less than or equal to extent Hb of the projected portion.