An interrogator is disposed in a main body of a vehicle for transmitting interrogating signals. A responder is mounted to each tire of the vehicle for returning tire information such as air pressure to the interrogator in response to the interrogating signals. The responder is provided with at least one sensor for detecting tire information and a sensor specifying means for specifying properties of the sensor. The interrogator is provided with a control means for processing tire information, and correction tables for correcting properties of a sensor specified by the sensor specifying means. The control means corrects the tire information by the correction tables and then outputs data related to the tire information.