It is prevented that magnetic foreign matter adheres the magnet encoder 3a to deteriorate the reliability of rotation speed detection with the sensor 4a. The encoder 3a is provided on the outside of the combined seal ring 19 with reference to the bearing interior space 24, and the sensor 4a faces the encoder 3a. The knuckle 21 is placed close to the housing 33 of the drive shaft member 28 of the constant velocity joint 27 to form a labyrinth seal 20. The width of the clearance of this labyrinth seal 20 is up to 40% of the pitch of adjacent S-poles and N-poles that are arranged on the detected surface of the encoder 3a. With this construction, larger foreign matter is prevented from attaching to the detected surface of the encoder 3a to solve the above subject.