PURPOSE: To surely detect the abnormal reduction of the internal pressure of a tire with a simple device by providing an infrared pyrometer on the tire, comparing the output of the pyrometer with a limit value, and generating an alarm when the output exceeds the limit value.

CONSTITUTION: An infrared pyrometer 11 is provided on each tire of a vehicle such as a racing car, and it is fitted to the chassis of the vehicle via a bracket 13 to face the inner shoulder of the tire, i.e., a target region 14 on the inside portion of a travel tread. The pyrometer 11 has a sensor 16 receiving the infrared rays 17 emitted from the target region 14 through a window 15 and outputs its analog electric signal to a common control unit 20. The control unit 20 monitors the temperature rise state of a tire group, operates a visible and/or audible alarm 27 in the overheated state that the tire temperature exceeds a limit value, and reports that the tire has a problem to a driver.