An image forming apparatus may include a casing, and a plurality of image carriers arranged in parallel along one direction in the casing and formed with an electrostatic latent image. The apparatus can further include a plurality of developing agent carriers opposed to the image carriers, for visualizing the electrostatic latent image by feeding a developing agent to the image carriers, and a plurality of developing agent cartridges, arranged in parallel in the one direction to be opposed to the developing agent carriers in the longitudinal direction of the developing agent carriers, and configured to be together attachable/detachable to/from the casing along the one direction, for accommodating the developing agent fed to the developing agent carriers.