PURPOSE: To alter tire air pressure according to traveling conditions without sucking and exhausting air from and into the atmosphere.

CONSTITUTION: A second chamber 4 capable of changing its volume due to a difference between its pressure and in the pressure a first chamber 3 is provided in the first air charged chamber 3 formed between a tire 1 and a wheel rim 2 for holding it. Also, a fluid, whose volume is increased as temperature rises, is sealed in the second chamber 4.