A tread rubber is made of a rubber composition containing 100 pbw of a diene rubber, 30 to 100 pbw of a reinforcing filler including at least 10 pbw of silica, and 1 to 20 pbw of microcapsules each encapsulating a heat-expandable substance. A sulfur-containing silane coupling agent is mixed in the rubber composition in an amount of 3 to 15 wt. % based on the weight of the silica. A shell material of each microcapsule is made of a thermoplastic resin which essentially contains a nitrile monomer. A vapor pressure of the heat-expandable substance is 1.4 to 3.0 MPa at 150° C. An average diameter of the microcapsules is 20 to 30 μm before vulcanization of the rubber composition. An average diameter of the microcapsules expanded due to the vulcanization is 40 to 80 μm. The tread rubber has a proportion of a cell-occupying area of 5 to 30%.