A color image forming apparatus which is capable of preventing deterioration in image formation quality caused by vibrations of a developing unit without halting an image forming operation and using a simple construction. A developing rotary unit incorporates developing devices corresponding to respective ones of a plurality of image formation colors. In forming an image, the developing rotary unit is moved to location for development of an image of each of the plurality of image formation colors. The level of vibrations applied to the developing rotary unit is detected, and the drive control pattern for the developing rotary unit is determined depending on the detected level of vibrations.