To effectively prevent the entry of foreign matter into a support unit and prevent grease, etc,. filled in the support unit from scattering outside.

This ball screw device 20 is provided with a screw shaft 3 having a spiral screw groove 2 on its peripheral surface, a ball nut 6 having a screw groove corresponding to the screw groove 2 of the screw shaft 3 on its inner peripheral surface to be loosely engaged with the screw shaft 3, many balls fitted between both grooves so that they can be rolled, and a support unit 7B wherein a rolling bearing 13 to rotatably support the end part of the screw shaft 3 is assembled. A peripheral groove 21 is formed on the outer periphery of the screw shaft 3 and a disc member 23 is mounted to the peripheral groove 21 to dispose the disc member 23 face to face with the support unit 7B.