A side-reinforcing rubber composition for a run flat tire, which achieves both low heat generating properties and high strength and improves run flat durability, and a run flat tire which uses such a rubber composition, said composition containing 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, 5 to 120 parts by weight of a laminated natural mineral having an aspect ratio of 3 to 30 and average particle size of 2 to 30 .mu.m, and at least 2 parts by weight of sulfur or a sulfur compound, based on 100 parts by weight of a diene rubber component.