A rubber composition for a run-flat tire comprises from 20 to 100 parts by weight of a reinforcing filler per 100 parts by weight of a diene rubber containing from 10 to 80 wt. % of butadiene rubber and from 10 to 40 wt. % of natural rubber; wherein the reinforcing filler contains at least 50 wt. % of carbon black; a mode diameter Dst in a mass distribution curve of a Stokes diameter of aggregates of the carbon black is at least 145 nm; a nitrogen adsorption specific surface area N2SA is from 45 to 70 m2/g; and a ratio N2SA/IA of the nitrogen adsorption specific surface area N2SA to an iodine adsorption IA (units: mg/g) is from 1.00 to 1.40.