PURPOSE: To make air pressure of a tire adjustable by communicating a closed space with the inside of the tire through a communicating hole formed in a turning wheel, and sucking and exhausting air to/from the closed space through an intake and exhaust hole of a knuckle or a fixed wheel.

CONSTITUTION: A pair of inner wheels 3 are engaged around a knuckle 2 fixed to a knuckle arm 1 constituting a suspension device for a wheel, and the respective inner wheels are sandwiched between a nut 4 driven into the edge part of the knuckle 2 and a step part 5 formed in a base edge part of the knuckle 2. Furthermore, an intake and exhaust hole 6 is formed in the inside central part of the knuckle 2. Meanwhile, a turning wheel 8 is supported around the respective inner wheels 3 through plural number of rolling elements 7 so as to be rotatable freely. Further, a closed space 17 capable of communicating with the outside only through the intake and exhaust hole 6 and a communicating hole 16 is formed between a sealing material 13 and a cover plate 11. In the main part 9 of the turning wheel 8, the communicating hole 16 is arranged in a condition capable of communicating the closed space 17 with the outside of the main part 9.