To provide a rubber reinforcing steel cord capable of minimizing the cord diameter and suppressing a dimensional change while ensuring sufficient adhesiveness between the cord and the rubber and to provide a pneumatic radial tire using the steel cord.
The steel cord 10 used in a belt layer 6 has a flat structure in which four side strands 12 having the same wire diameter as the core strand 11 are stranded and arranged on the outside of the one core wire 11 subjected to helical preforming and the circumscribed circle C11 of the core strand 11, and the circumscribed circle C12 of the side strands 12 form an elliptical shape, respectively. The twisting direction of the core strand 11 is the same as the strand direction of the side strands 12 and the preforming pitch of the core strand 11 is the same as the strand pitch of the side strands 12. The relationship between a minor axis ds of the circumscribed circle C11 of the core strand 11 and the wire diameter d satisfies ds>d. The minor axis Ds of the circumscribed circle C12 of the side strands 12 satisfies 2d
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Takashi Noguchi
Kazuhiko Saishita