A method of manufacturing a radial tire is disclosed, wherein a tread ring is formed separately from a tire main body by the use of a tread ring forming apparatus. The radial tire comprises a belt having a camber and a band wound thereon. The tread ring is an annular assembly of a raw belt, a raw band wound thereon and a raw tread rubber. The tread ring forming apparatus comprising a belt drum made up of segments defining the drum circumference. In a cross section including the belt drum axis, the radially outer surface of each segment is a straight line parallel to the axis. In a cross section normal to the belt drum axis, the radially outer surface is an arc having one radius substantially equal to one half of the drum diameter, but the drum circumference is decreased in the belt edge portion than the central portion by increasing gaps between the segments.