To provide a rotational speed detection device miniaturized in an axial direction and a rolling bearing device using it.

In the rotational speed detection device fixed to the fixing side sealing device which is fixed to a fixing member, the shape of the projection 26 projecting to axial direction and an outer peripheral direction of the fixing member is made a shape having protruding part 80. Moreover, the amount of projection is made small without reducing the bending stress in the axial direction so that the rotational speed detection device is made minimized in the axial direction. Furthermore, by embedding the reinforcing member in the projection part 26, it is further miniaturized in the axial direction.