A tandem type color image forming apparatus has a plurality of image forming units. These units includes at least a plurality of spaced photosensitive drums, and a plurality of developing rollers for developing with toners electrostatic latent images formed on these photosensitive drums. The image forming units except the image forming unit for black color that may be highly frequently used are accommodated in the same first housing. The unit for black color is accommodated in another second housing. The first and second housings are made to be attachable to and detachable from the chassis of the apparatus main body independently of each other. Only an image forming unit with high frequency of inspection or maintenance can separately be detached, so that the work load for inspection or maintenance is reduced, and handling of the apparatus is easy.