A transmitting device which is disposed in a tire hollow region, and which transmits tire information relating to the state of the tire, comprises: a sensor which detects, as tire information, a state of a gas which is filled in the hollow region of a tire which is surrounded by the tire and a rim; a transmitter which wirelessly transmits the tire information; and a casing which houses the sensor and the transmitter therewithin. The casing further comprises a through hole which passes through a wall of the casing, and which extends in an orthogonal direction to a sensor detection face of the sensor. The surface area of an outer side aperture part of the through hole is 0.8 mm2 or less, and the sensor detection face faces an inner side aperture part of the casing inner side of the through hole without obstructing with the space.