A color image forming apparatus provided with a developing device for developing an electrostatic latent image formed on a latent image bearing member. The developing device comprises a plurality of developing units and supporting members for supporting the developing units thereon, and an arrangement is such that any developing units can be installed detachably on any supporting members each other. Then, the developing units have a developer supplying mechanism for feeding a developer each, and a supplemental developer container can be connected to each developer supplying mechanism. Then, the supplemental developer container is connected to one developing unit, and the developing unit provided with the supplemental developer is installed on a predetermined supporting member. The remaining developing units free from the supplemental developer container are then installed on the remaining supporting members. Accordingly, by connecting the supplemental developer container to a developing unit of the color to be consumed relatively more than the others and installing such developing unit on a predetermined supporting member, the developer of a desired color can be contained highly satisfactorily without constructing the color image forming apparatus entirely in a large size.