Rotation torque of the hub rotating together with the road wheel is reduced to improve the driving performance of the vehicle such as acceleration performance, fuel consumption.The opening at both ends of the space where balls 14, 14 are provided is covered by seal rings 16c, 16d each having two or three seal lips. The whole rotation torque of the both seal rings 16c, 16d based on the friction between the seal lips and the mating surface is reglated between 0.06 Nm and 0.4 Nm.