To realize a structure for preventing axial distortion and deformation in the inner-diameter-side of a flat plate of a cover for closing the axial inner end opening of an inner space installed with an encoder according to inserting/fixing a cylindrical part of the cover to the axial inner end of an outer ring.

The structure has an outer diameter smaller than the inner diameter of the axial inner end of the outer ring 7 at an axial inner end of the cylindrical part 20. A non-contact part 26a, having an axial dimension L26 twice as large as the thickness t19 of a plate member composing the cover 19a or larger, is formed over the whole circumference. In a state that the cover 19a is inserted to the axial inner end of the outer ring 7, the cylindrical part 20 is inserted to the axial inner end of the outer ring 7 only at an axially outer part rather than the non-contact part 26a by tight fitting.