An improved wheel bearing unit is composed of an outer race member having two rows of raceway surfaces formed in its inner peripheral surface and attached to the body of a motor vehicle, and an inner race member provided with a wheel mounting flange and rotatably supported through two rows of balls by the outer race member. In the vicinity of the groove bottom portion of the raceway surface of the inner race member is provided a projection having a diameter greater than the inscribed circle diameter. The raceway surface, the outer peripheral surface of the projection, and the smaller end face are concurrently ground to keep within a predetermined range of standard values the radius difference between the diameter of the groove bottom portion of the raceway surface of the inner race member and the outer diameter of the projection, as well as the core deviation between the groove bottom portion of the raceway surface and the smaller end face.