PURPOSE: To move and hold the developing machine correctly on a developing device by coupling a cam shaft with a driving source through a spring clutch whose rotational position is controlled and driving the driving source only when the developing machine is moved.

CONSTITUTION: The cam shaft 99 is coupled with the driving source through the spring clutch 102 whose rotational position is controlled, and the driving source is driven only when the developing machine is moved. Therefore, a spring 112 does not wear out by coming into slide contact with the driving side 110, so the position where the rotation of the spring clutch 102 is controlled becomes invariably constant and the stop positions of cams 100 and 101 become invariably constant. Consequently, the developing machine is moved to a development position with accuracy at all times.