PURPOSE: To reduce the weight and improve the lateral rigidity, durability, and controllability, by burying a steel cord of specific sectional form perpendicular to the lengthwise direction of the cord in which three wires and six wires of larger diameter than the former are twisted together in the same direction and at the same pitch into 1×9 structure in a belt layer.

CONSTITUTION: A ply constituting a belt layer buried in a crown part under a tread is formed between rubber matrixes 5 so that steel cords 6 are buried at regular intervals and angle and in parallel with each other. The steel cord 6 is formed into 1×9 structure by twisting together three core wires 7a-7c and six side wires 8a-8f of larger diameter than the former in the same direction and at the same twisting pitch, the circumscribed circle of the section perpendicular to the cord axis is flat, and the degree of flatness is set so that D1/D2 is 1.05-1.20 where major and minor axes are respectively D1, D2. Consequently, permeabilty of rubber into the steel cords 6 and fatigue resistance are good, durability is high without slip out of the cord core, weight is reduced, lateral rigidity is improved, and cornering characteristics can be improved.