Cartridge for image forming apparatus

To provide an image forming apparatus capable of increasing the flexibility of installation place, and to provide cartridges that are attached to the image forming apparatus. The main body casing 4 of a printer 2 is composed of a lower casing 8 and an upper casing 9 that opens or closes relative to the upper face 107 of the lower casing 8. A cartridge attachment/detachment openings 34 are provided in the upper face of the lower casing 8. The lower casing 8 has guide members 31, with which process cartridges 25 are guided so as to be attached or detached along the curved attaching/detaching paths, extending along the prescribed face outside the lower casing 8 across the cartridge attachment/detachment openings.