A rolling bearing unit with rotating speed sensor comprising a stationary outer ring member having an outer ring raceway, a rotatable inner ring member having an inner ring raceway, a plurality of rolling elements rotatably provided between the outer ring raceway and the inner ring raceway, a cylindrical magnetic tone wheel fixed to the inner ring member and, having a cylindrical fixing portion fitted onto the inner ring member, a cylindrical detecting portion provided concentric with the cylindrical fixing portion and formed with rotating cutouts, and a stepped portion provided between the cylindrical fixing portion and the cylindrical detecting portion, and a cover comprising a synthetic resin block and a metal portion to be securely mounted to the outer ring member, and an annular sensor embedded in the synthetic resin block so as to be opposed to the tone wheel, and the cylindrical fixing portion having an inner peripheral face on which a plurality of engagement portions are arranged in a single circle to project radially inwards from the inner peripheral face, and to be abutted to the end face of the inner ring member, so that the tone wheel is positioned with reference to the inner ring member.