To provide a method to cool sufficiently a wheel brake device even if a wheel is mounted in which the brake device has a poor cooling environment and provide a wheel constitution for cooling the brake device efficiently.

An in-wheel motor system according to this invention includes an in-wheel motor 3 of hollow shape which is installed in each wheel part and supported through a buffer device 10 on a knuckle 5 as an underbody component of the vehicle, wherein a heat conducting member 9 made from a material having high thermal conductivity such as silicone grease or silicone series rubber, graphite, aluminum alloy, or copper alloy is interposed in the mounting boundary surfaces of the wheel 2 and the brake device 8 where the rotary part of the motor 3 is installed, and the heat generated by the brake device 8 is conducted to the wheel 2 through the heat conducting member 9 so that the brake device 8 is cooled efficiently.