To use an encoder 6b to prevent the entry of rainwater or others from a fitting face between a slinger 5 and an inner ring 4 into a space 17 and prevent the axial projection of a portion of an encoder 6b from the end face of the inner ring 4.

The inner periphery edge of the encoder 6b is provided with a tongue part 19 all over the periphery which elastically abuts on the outer periphery face of the inner ring 4 in the state of being elastically deformed concurrently with the abutment on the outer periphery face of the inner ring 4. A portion near the inner periphery edge of the encoder 6b is provided with a recessed part 21. When the tongue part 19 abuts on the outer periphery face of the inner ring 4, the elastically deformed portion of the tongue part 19 is stayed into the recessed part 21 and prevented from projecting from the side face of the encoder 6b and the end face of the inner ring 4.