A composition formed by compounding from 55 to 75 parts by mass of a carbon black (1) having a nitrogen adsorption surface area of at least 35 m2/g and less than 50 m2/g and from 5 to 20 parts by mass of a carbon black (2) having a nitrogen adsorption surface area of at least 50 m2/g and at most 95 m2/g with 100 parts by mass of a rubber component including from 30 to 70 mass % of butadiene rubber having a cis-1,4 bond content of at least 97% and a Mooney viscosity (ML1+4) of at least 45 at 100° C., a ratio (T-cp)/(ML1+4) of the viscosity of a 5 mass % toluene solution at 25° C. (T-cp) [cps] to the Mooney viscosity being at least 2.0, and from 30 to 70 mass % of other diene rubbers; the total of the carbon blacks (1) and (2) being from 60 to 95 parts by mass.