To detect tire pressure with precision by supporting a moving magnet at the outer periphery of a rim and moving it along the periphery of the tire according to tire pressure variation, providing a fixed magnet on its extension line, and detecting mutual relating positions of magnets for finding the tire pressure.

A magnet assembly is provided on a rim 2a in a tire 1, and magnetism detectors 4 and 5 are provided in the rotation direction, while facing each other. The assembly 3 has a moving magnet 15 moving in the rotation direction based on the balance between a fixed magnet 8, fixed along the rotation direction and the pressure in the tire 1, a bellows 12 and a coil spring 16. By rotation of the tire 1, two signal waveforms due to passage of the magnets 8 and 15 are generated in the detectors 4 and 5. The angle at a reference pressure of the magnets 8 and 15 viewed from the center of the tire 1 is made equal to the angle formed with the detectors 4 and 5. So that, the time between zero cross points of the magnets 8 and 15 is measured for finding the shift amount of the magnet 15 from the position at the reference pressure, and it is converted into the pressure of the tire 1.