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.