The present invention provides a rubber composition for a tire, which achieves both low heat generating properties and high strength and improves durability of a run flat tire, and a run flat tire obtained using the rubber composition. Specifically, the present invention provides a rubber composition for a tire comprising 10 to 100 parts by weight of carbon black having nitrogen-adsorbing specific surface area of 30 to 100 m.sup.2/g and dibutyl phthalate oil absorption of at least 50 ml/100 g, and at least 2 parts by weight of a vulcanizing agent, based on 100 parts by weight of a rubber component containing 10 to 50% by weight of star-shaped solution polymerization butadiene rubber, and a run flat tire using the composition as the reinforcing rubber layer of the sidewall.