PURPOSE: To arrange a sensor rotor and a pickup sensor without interfering with peripheral members even when the gap between the end face of hub on the knuckle side and the side face at the knuckle of a brake rotor is limited.

CONSTITUTION: A sensor rotor 7 is provided, on the outer circumferential face thereof, with irregularities 71 extending in parallel with the axis of a drive shaft S. The sensor rotor 7 is fitted over a hub 4 at the end part on the knuckle 12 side thereof. A pickup sensor 8 is fixed with the longitudinal direction of detecting section 82 inclining against a plane intersecting the drive shaft S perpendicularly (a plane perpendicular to a reference line L or the plane of drawing including a line extending in parallel with the reference line L). The detecting section 82 is beveled, at the forward end face thereof, in parallel with the line of irregularities 71 on the sensor rotor 7.