A rubber composition for a tire containing (i) 100 parts by weight of a diene-based rubber and (ii) 0.5 to 25 parts by weight of a heat-expandable microcapsule including a shell, and a substance capable of vaporizing or expanding under heating to thereby generate a gas and a nonpolar oil, both encapsulated in the shell, wherein the shell is made of a thermoplastic resin obtained by polymerization of a nitrile-based monomer (I), a monomer (II) having an unsaturated double bond and a carboxyl group in the molecule thereof, an optional monomer (III) having two or more polymerizable double bonds, and an optional copolymerizable monomer (IV) for adjusting the expansion properties.