PURPOSE: To regulate air pressure of a tire arbitrarily without feeding air from the outside by providing a pressurizing body disposed in the air in a tire, which is made of shape memory alloy which can transform itself from the condition of a decreased volume to that of an increasing volume reversibly.

CONSTITUTION: When the switch 26 of an electromagnet 16 is turned on during driving a vehicle, a magnetic field is generated by the magnetic body 24 of the electromagnet 16 and a coil 30, and the angular velocity of a coil 18, that is to say, alternating current in proportion to vehicle speed is obtained. When the alternating current reaches a certain value, a pressurizing body 22 transforms itself by heat generation of a heat generation device 20 to increase its volume. Consequently, the air in a tire 14 is compressed to increase air pressure. On the other hand, when the vehicle speed drops and the current goes down, or the switch 26 is turned off, the volume of the pressurizing body 22 decreases, thus air pressure in the tire 14 returns to its original magnitude.