To prevent accident in high-speed running caused by shortage of the air pressure in a vehicle tire by automatically adjusting the requisite air pressure to a proper value in accordance with the current running speed.

In recent years, a vehicle tire is of tubeless type chiefly, so that a high pressure air is obtained easily by installing an air pump S in the wheel A and utilizing the vehicle weight to the power for the pump S. For example, five pumps of 0.2-cc capacity per piece are installed on each wheel, and when the wheel is rotated one turn, an air quantity of 1 cc can be fed to the tire upon being compressed.