A rotating shaft is accommodated in a case. A ferromagnetic portion is formed on the rotating shaft, and electromagnets are provided to the case. Many projecting portions are formed so as to be arranged in a direction along which the movement of the rotating shaft is required to be regulated. Furthermore, Many projecting portions are likewise formed on the ferromagnetic portion. According to this construction, magnetic flux occurring in the electromagnets concentrates, so that restoring force occurs in the axial direction with suppressing reduction of the attractive force in a radial direction to the ferromagnetic portion. Therefore, the movement in the axial direction of the rotating shaft can be regulated.