To bury a passive type radio tag in a tire, and to simultaneously send a tire ID memorized in the radio tag to an exterior device beyond a communication range of the radio tag with air pressure data.

A pressure monitoring unit 4 is attached to a tire inner space 11 side of an air valve 3. In the inside 12 of a structure member of the tire 1, the radio tag 5 is buried. When a radio communication device (not shown) on the outside accesses to the pressure monitoring unit 4, the pressure monitoring unit 4 detects air pressure of the tire inner space 11, and accesses to the radio tag 5. The radio tag 5 operates by using electric power of an access signal of the pressure monitoring unit 4 as a poser source, and sends the tire ID memorized in advance to the pressure monitoring unit 4. The pressure monitoring unit 4 simultaneously sends the received tire ID, and the detected air pressure data to the radio communication device on the outside.