In order to achieve a structure that can ensure sufficient sealing performance of a cap (19a), in a state in which a bottomed cylindrical fitting core (44) made from a non-magnetic material is fixed radially inside of a cap cylindrical portion (22a) which configures a bottomed cylindrical cap main body (20a) made from a synthetic resin, an axial outer end opening of a through-hole (25a) formed in the cap bottom (23a) of the cap main body (20a) is closed by the axially inside surface of the core bottom (48) of the fitting core (44). Then, the outer peripheral surface of the core cylindrical portion (47) of the fitting core (44) is directly fitted inside of, and fixed to, the inner peripheral surface on the axially inside end of an outer ring (2).