A method of fastening a sub frame to a car body by a fastening bolt, in which the car body is provided with a screw hole for the fastening bolt and with an introduction hole for the fastening bolt, the sub frame is provided with a through-hole for the fastening bolt, and the introduction hole and the through-hole are provided with a clearance for inserting the fastening bolt, respectively, characterized in that a sub frame rigid collar is provided with a collar body which includes a fastening bolt through-hole having a larger diameter than an outer diameter of the fastening bolt, a collar provided to and projected from the outer periphery of the collar body, wherein an upper outer peripheral portion of the collar body is provided with an upper peripheral edge portion having its outer diameter smaller than an inner diameter of the introduction hole of the car body, and an upper tapered outer peripheral portion at the car body side of which the diameter gradually grows from the upper peripheral edge portion toward an upper collar root tip peripheral edge portion of the collar body, and in which an outer diameter of an upper outer diameter portion provided at lower side from a center section of the upper tapered outer peripheral portion i or at said center section s larger than the inner diameter of the introduction hole of the car body, a lower outer peripheral portion of the collar body is provided with a bottom peripheral edge portion having an outer diameter smaller than an inner diameter of the through-hole of the sub frame, and a lower tapered outer peripheral portion at the sub frame side of which the diameter gradually grows from the bottom peripheral edge portion toward a lower collar root tip peripheral edge portion of the collar body, and in which an outer diameter of a lower outer diameter portion provided at an upper side from a center section of the lower tapered outer peripheral portion or at the center sections larger than the inner diameter of the through-hole of the sub frame, the sub frame rigid collar integrally formed with a metal having substantially a plasticity of aluminum or of a thermal refining material equal therewith, is inserted into the introduction hole of the car body and the through-hole of said sub frame while the threaded portion of the fastening bolt is inserted into the fastening bolt through-hole of said sub frame rigid collar, and interposed between the car body and the sub frame, and after that, the fastening bolt is fastened by a necessary torque while screwing the threaded portion of the fastening bolt in the screw hole of the car body, the upper tapered outer peripheral portion at the car body side and the lower tapered outer peripheral portion at the sub frame side are fastened, by compressing and force-fitting the outer diameter portions (21d, 22d) of the collar body having its diameter larger than the inner diameters of the introduction hole and the through hole respectively while deforming the outer diameter portions (21d, 22d) with the plasticity of the sub frame rigid collar, thereby, press-attaching and press-fitting the outer diameter portions (21d, 22d) between the inner peripheral surface of the introduction hole of the car body and the through hole of the sub frame and the outer peripheral surface of the fastening bolt, and by intervening and press-attaching the collar between the surface of portal edge portions of introduction hole of the car body and of the through-hole of the sub frame.