<p>Provided is a tire air-pressure sensor (30) for making water hard to flow into a communication passage (36) communicating with the sensor portion (18a) of an air-pressure sensor element (18), and for discharging the water having invaded the communication passage (36), to the outside of a case (32). Two air holes (34) and (34) are so formed in the crest portion of the case (32) of the tire air-pressure sensor (30) arranged and fixed in a wheel (10) as are positionally shifted in the rotating direction of the wheel (10). An air vent hole (16) has its opening formed in the rotating direction at an intermediate position between the two air holes (34) and (34) in a cap (38) to cover the air-pressure sensor element (18) arranged in the case (32). The two air holes (34) and the opening of the air vent hole (16) are made to communicate via the communication passage (36) between the crest portion of the case (32) and the cap (38) and substantially in a plane normal to the radial direction of the wheel (10). Moreover, a wall (40) is interposed between the air holes (34) and (34) and the air vent hole (16) so that the communication passage (36) may have a meandering state.</p>
