The offset amount that a sacrificial rib road contact surface is recessed to the inner side of a tire with regards to a shoulder rib road contact surface is equal to 0, and a circumferential fine groove is provided such that the opening side thereof is located on the inner side when the tire is filled to a proper internal pressure. The dimension Ws in the tire width direction of the shoulder rib road contact surface and the dimension Wa in the tire width direction of the sacrificial rib road contact surface are set such that 0.10≦Wa/Ws≦0.2. The dimension Wb in the tire width direction of the sacrificial rib road contact surface when the groove depth of the circumferential main grooves has worn 20% is set in a range such that 0.25≦Wb/Ws≦0.30.