PURPOSE: To move and maintain a moving body to and in accurate prescribed positions in a lens changer or the like for an unmagnification and variable power copying machine, by changing the moving direction of the moving body after said body goes out of a differential range of a position detecting means when a change signal for the moving direction is inputted.

CONSTITUTION: A driving screw shaft 5 which is rotated and driven by a driving motor 4 which can run reversely is provided. Lens housing 3 mounted to a lens 1 for unmagnification and a lens 2 for variable powers is screwed to a screw shaft 5, and is supported freely slidably to a guiding rod 6 provided in parallel with the shaft 5. The lenses are changed when the housing 3 is moved. When the user pushes an unmagnification key right after pushing a variable power key, and if the housing 3 is still moving in the variable power direction within the differential range of an unmagnification position switch 7, the housing is kept moved in the variable power direction until the same goes out from the differential range and where a switch 7 goes off upon going out of the differential range, the motor 4 runs reverse and the housing 3 begins to return in the unmagnification direction. Thus the housing 3 is moved in the prescribed positions irrespectively of the differential range.