The outer race 2 is fixed to the support hole 29 in the knuckle 1, and the constant velocity joint 6 is connected to the hub 3. The first seal ring 9 is fitted into the outer race 2 to cover the opening on the end of the bearing space 26. The seal ring 28 is fitted into the support hole 29 to cover the opening on the end of the detection space 27. The encoder 7 is fitted onto the inner race 4, and the outer peripheral edge of the encoder 7 faces the inner peripheral surface on the outer race 2 in an adjacent arrangement relationship to form a labyrinth clearance 36, so that grease within the bearing space 26 is prevented from leaking toward the detection space 27, whereby the rotation resistance is small to improve parts of the automobile in acceleration and fuel consumption.