A casing of a ball bearing which holds a ball screw can be displaced on a supporting stand against urging force of a compression coil spring and a tensile coil spring. If vibration of the ball screw is transmitted to the casing, the casing attempts to move on the supporting stand. However, when, due to the vibration, the casing is displaced against the urging force of the compression coil spring and the tensile coil spring, the urging force forcibly attempts to return the casing to its original position. A frequency of vibration of the casing is thereby forcibly changed.