There is provided a run flat tire having a side reinforcing layer in which low exothermic property and high hardness are compatible and durability is improved. A rubber composition containing 10 to 100 parts by weight of carbon black with a nitrogen adsorption specific area of 30 to 100 m.sup.2/g and a dibutyl phthalate oil absorption amount of 50 ml/100 g or more and 2 parts by weight or more of sulfur or a sulfur compound, based on 100 parts by weight of a rubber component containing 10 to 50% by weight of a conjugated diolefin polymer or a copolymer of a diolefin and an aromatic vinyl compound having a primary amino group and an alkoxysilyl group and having a glass transition point of -30.degree. C. or less in which a bifunctional or more monomer is copolymerized in a polymer chain and/or at least a portion of a (co)polymer chain is coupled with a bifunctional or more coupling agent, and a run flat tire having a side reinforcing layer comprising the rubber composition.