The viscosity of a puncture-preventing sealant (7) is set in a range of 25 to 35 dPanulls, and a thixotropy index is set in a range of 6.5 to 11.0, the sealant (7) being filled in a sealant chamber (6) defined to extend along an outer periphery of an air chamber (5) in a tire (T). When the tire (T) is being rotated by traveling of a vehicle, the viscosity of the sealant (7) is reduced by a stirring action provided by a vibration or a centrifugal force of the sealant (7), whereby the sealant (7) can be diffused uniformly in the sealant chamber (6). When the vehicle has been stopped and the stirring action has been extinguished, the viscosity of the sealant (7) is increased, whereby the sealant (7) is maintained in a state in which it has been diffused uniformly in the sealant chamber (6) without flowing downwards in the sealant chamber (6) by gravitation. Therefore, even when the vehicle is travelling at a high speed, the generation of a vibration and a noise can be prevented. In this manner, the puncture-preventing sealant is provided, which has such a nature that the sealant is easy to flow due to a low viscosity during traveling of a vehicle, and the sealant is difficult to flow due to a high viscosity when the sealant is left to stand.