A rubber composition including 100 parts by mass of a diene rubber containing natural rubber, and blended with from 40 to 80 parts by mass of carbon black, cobalt neodecanoate borate represented by the following Chemical Formula (1), a phenolic resin, a curing agent, sulfur, and a vulcanization accelerator, wherein the carbon black has DBP oil absorption of from 50×10−5 to 80×10−5 m3/kg, and iodine adsorption of from 100 to 150 g/kg, and a vulcanized rubber has characteristics of a dynamic storage modulus (E′) at a dynamic strain of 2% and at 20° C. of 8 MPa or more, a tangent loss (tan δ) at 60° C. of 0.20 or less, and the number of times of repetition until fracture is caused in a constant strain fatigue test at a strain of 60% and at 400 rpm of 45,000 or more.