TRANSMISSION DEVICE AND TIRE STATE MONITORING SYSTEM

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 mm 2 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. A distance (h mm) which is the length of the through hole is 3 to 15 mm, and a ratio A/V of a surface area A mm 2 of the lateral wall of the through hole to a volume V mm 3 of the through hole is 3.0 to 30.0 mm -1 .