A valve-integrated transponder includes an air valve and a casing that are integrated with each other, and is disposed within a tire. The casing houses a substrate having a detecting element mounted thereon, an inverse F antenna element, and a supporting member. The antenna element has a substantially line symmetrical shape where a pair of radiation conductors extend away from each other. A longitudinal direction of the substrate is substantially perpendicular to an axial direction of the air valve, and a lateral direction of the substrate is inclined relative to a well of a wheel rim. The two radiation conductors of the antenna element disposed on the substrate are inclined at a predetermined angle with respect to the substrate, so as to be disposed substantially in parallel to the well that extends at the inner side of the transponder as viewed in the radial direction of the tire.