A radial force variation of a tire is corrected irrespectively of the kind of the organic fiber cord used, wherein a radial force variation (RFV) is measured at room temperature to specify a tire position indicating a minimum value of RFV, and an inner face of a tread portion is pushed and enlarged outward in a radial direction of the tire by a pushing means at a RFV bottom region sandwiching the position of the minimum RFV value from both sides thereof in a circumferential direction of the tread portion, and such a push enlarged state is kept for a given time.