The present invention aims to improve the efficiency of a clamping step for combining a flange with a shaft and to improve productivity. The shaft and the flange are tentatively combined in the tentative clamping step. In the tentative clamping step, a concave part at the end of the shaft is pressurized by a metal mold such as a ball to be enlarged in an outer circumferential direction, thereby pressurizing this concave part against the inner circumference of the flange so as to fix the concave part. The combined body made by tentatively combining the shaft and the flange is strongly combined in a proper clamping step. In order to correct a distortion such as a warpage of the flange that has been solidly combined by the proper clamping step, sandwiching the flange by the upper and lower metals, the flange is pressurized and a flash molding is carried out.