When a sidewall portion is formed with a white line or character having different color from that of a main body rubber, a rubber ribbon is wound along a circumferential direction of the tire a plurality of times, a black sidewall rubber provided with a concave groove extending in the circumferential direction of the tire is formed in a region where the white line or character is to be formed, an integrally extruded white rubber extrudate is fitted into the concave groove, the white rubber extrudate is covered with a black rubber sheet, the rubber sheet is partially cut down to expose the white rubber extrudate.