To provide a cost-reduced rotation supporting apparatus for wheels in which not only invasion of a foreign substance through a portion between a mounting part 4a and a backing plate 2a but also deposition of the foreign substance on an encoder 20 can be prevented and further, a friction loss can be suppressed.

A mounting part 4a is provided in an inner terminal portion of an outer ring 6a in the axial direction, and an inner side surface of the mounting part 4a in the axial direction is positioned innermost with respect to the axial direction in the outer ring 6a and a hub 8. Furthermore, a bottomed cylindrical cover cap 35 made of a nonmagnetic plate is fitted into a portion closer to an inner edge of the outer ring 6a in the axial direction, and an encoder 20 and a sensor 22 are confronted in the axial direction via the bottom of the cover cap 35. Moreover, a backing plate 2a is formed approximately in a disc shape which does not have a through hole in the center, thereby closing an inner edge opening of the outer ring 6a in the axial direction with the backing plate 2a. Thus, a contact combination seal ring can be omitted.