To facilitate the mounting of a magnet and a magnetic sensor on a tire side and a body side and improve the precision of vehicle speed detection by attachably and detachably mounting the magnet on the air injection plug of a vehicle tire, and mounting the magnetic sensor for detecting the magnetic field generated by the magnet in vehicle traveling.

A male screw is formed on the circumference of an air injection plug 5 provided on the inside of a tire 2 so as to protrude, and a valve lid 6 consisting of a cylindrical part 6a and a top part 6b for blocking the opening end is screwed to the male screw. A magnet 3 having a high magnetic force is attachably and detachably mounted on the top surface of the valve lid 6 through an adhesive 8. On the other hand, a magnetic sensor consisting of a Hall element or magnetic resistance element for detecting the magnetic field generated by the magnet 3 in vehicle traveling is mounted on the body near the tire 2, for example, on the lower part of a rear fender. The change of the external magnetic field is detected by use of the change of impedance, so that the vehicle speed can be precisely detected from the output of the magnetic sensor 4.