PURPOSE: To enlarge the field of application with increase in the function and to make the whole apparatus compact by forming a flat surface parallel to the axis of a ball screw shaft, and engaging a rotation preventing portion of a rotation transmitting member mounted on a housing with the flat surface.

CONSTITUTION: A flat surface 3 parallel to the axis is formed on a ball screw shaft 1, and a rotation preventing portion of a rotation transmitting member 24 mounted on a housing 6 is engaged with the flat surface 3. When a ball screw nut 5 is rotated by an electric motor 12, with the rotation transmitting member 24 stopped, the ball screw shaft 1 is moved linearly. When the rotating speed of the ball screw nut 5 and the rotation transmitting member 24 is changed, the ball screw shaft 1 makes a compound motion of linear motion and rotation. Thus, the field of application can be enlarged by increasing the function, and the whole apparatus can be made compact.