51a, 51a and the inner raceway surface 55a, 56a, respectively, of the wheel hub 55 and the inner ring 56. The balls are rollably held by cages 58, 58. In addition, seals 59, 60 are mounted in annular openings formed by the outer member 51 and the inner member 52. The seals 59, 60 prevent leakage of grease contained in the bearing and the entry of rain water or dusts from the outside.
The constant velocity universal joint 61 includes an outer joint member 62, joint inner ring 63, a cage 64 and torque transmitting balls 65. The outer joint member 62 has a cup-shaped mouth portion 66, a shoulder portion 67, and a shaft portion 68. The shoulder portion 67 forms a bottom of the mouth portion 66.The shaft portion 68 axially integrally extends from the shoulder portion 67. The shaft portion 68 is formed, on its outer circumference, with a serration 68a to engage the serration 55c of the wheel hub 55. An outer screw thread 68b is formed on the end of the serration 68a. The outer joint member 62 is inserted into the wheel hub 55, via serrations 55c, 68a for torque transmission. The wheel hub 55 and the outer joint member 62 are united by a securing nut 69 fastened onto the outer screw thread 68b. 
As shown in FIG. 6, the seal 60 is formed by a so-called pack seal. It is a combination of a first sealing ring 70 and a second sealing ring 71. The first sealing ring