A bearing unit has a sensor for sensing rotating speed of a wheel. A concave or a convex portion is formed on an end surface of a wheel shaft rotating with a rotating inner ring member. A disk-like sensor rotor forms on its center a convex or a concave portion which is fit to said concave or convex portion of the wheel shaft.
A sensor is attached to a lid body covering an end portion opening of an outer ring member. Install length of the sensor rotor becomes small so that the length of the entire bearing unit becomes small.