A system comprising the combination of a hub unit, an annular cover element attached to an outer ring of the hub unit and having a holding portion, a sensor having a head portion adapted to be pushed into the holding portion, and a pulser ring attached to the inner ring of the hub unit, wherein a wheel is supported rotatably by the inner ring of the hub unit which is fixed to a vehicle body via the outer ring, wherein the head portion of the sensor for detecting a change in rotation of the pulser ring which is fixed to a vehicle inner side end portion in an axial direction of the inner ring of the hub unit is pushed into the holding portion which is provided on the annular cover element which is fixed to the outer ring of the hub unit to confront the pulser ring from a radial direction so as to be fixed therein, wherein the holding portion of the annular cover element has a guide piece for guiding the head portion of the sensor which is being pushed thereinto along the push-in direction on a vehicle outer side in the radial direction and a resilient piece which is pressed from the vehicle inner side so as to be brought into engagement with an engagement portion which is formed on the head portion, whereby the holding portion holds the head portion in the axial direction, wherein swelling portions which swell outwards are formed individually on both sides of the head portion in the push-in direction, wherein receiving portions for positioning the head portion in the radial direction by receiving distal end faces of the swelling portions in the push-in direction in the vicinity of the engagement position with the engagement portions are formed individually on both sides of the resilient piece in the push-in direction to project therefrom, wherein a receiving position where the distal end faces of the swelling portions are received by the receiving portions is situated further forwards in the push-in direction than the engagement position between the resilient piece and the engagement portion, and wherein the head portion of the sensor is pressed in the axial direction by a pressing action of the resilient piece and is pushed forwards in the push-in direction so as to be held in place while being received by the receiving portions to thereby be positioned in the radial direction, wherein the receiving portions are formed by bending both side portions of a resilient functioning portion of the resilient piece towards the vehicle outer side along bending lines in the push-in direction, to define flat surfaces extending along a direction vertical to the surface of the pulser ring, and wherein the distal end faces of the swelling portions are adapted to be received by the flat surfaces such as to position the head portion in the radial direction. wherein a wheel is supported rotatably by the inner ring of the hub unit which is fixed to a vehicle body via the outer ring, wherein the head portion of the sensor for detecting a change in rotation of the pulser ring which is fixed to a vehicle inner side end portion in an axial direction of the inner ring of the hub unit is pushed into the holding portion which is provided on the annular cover element which is fixed to the outer ring of the hub unit to confront the pulser ring from a radial direction so as to be fixed therein, wherein the holding portion of the annular cover element has a guide piece for guiding the head portion of the sensor which is being pushed thereinto along the push-in direction on a vehicle outer side in the radial direction and a resilient piece which is pressed from the vehicle inner side so as to be brought into engagement with an engagement portion which is formed on the head portion, whereby the holding portion holds the head portion in the axial direction, wherein swelling portions which swell outwards are formed individually on both sides of the head portion in the push-in direction, wherein receiving portions for positioning the head portion in the radial direction by receiving distal end faces of the swelling portions in the push-in direction in the vicinity of the engagement position with the engagement portions are formed individually on both sides of the resilient piece in the push-in direction to project therefrom, wherein a receiving position where the distal end faces of the swelling portions are received by the receiving portions is situated further forwards in the push-in direction than the engagement position between the resilient piece and the engagement portion, and wherein the head portion of the sensor is pressed in the axial direction by a pressing action of the resilient piece and is pushed forwards in the push-in direction so as to be held in place while being received by the receiving portions to thereby be positioned in the radial direction, wherein the receiving portions are formed by bending both side portions of a resilient functioning portion of the resilient piece towards the vehicle outer side along bending lines in the push-in direction, to define flat surfaces extending along a direction vertical to the surface of the pulser ring, and wherein the distal end faces of the swelling portions are adapted to be received by the flat surfaces such as to position the head portion in the radial direction.