Abstract: A detection system for detecting abnormality in the internal air pressure of a tire of a motor vehicle produces an alarm upon detection of an abnormal change in the air pressure of the tire. The system comprises a first device which is disposed at a wheel (or a rotating part) of the motor vehicle and has an abnormal pressure detecting means using a crystal resonator and an antenna for sending out detection information in the air; and a second device which is disposed for example in the vicinity of a driver's seat (or at a non-rotating part) and has a transmitter/receiver arrangement provided with an antenna. The crystal resonator is driven by an exciting energy sent out from the transmission part of the transmitter/receiver.