A developing unit is detachably mounted on a photosensitive unit to form a combined unit called a process unit. In use, the process unit is loaded into an electrophotographic image forming device. A rotational shaft of a photosensitive drum projects out from the photosensitive unit. When the process unit is loaded into the image forming device from a top open space, both ends of the rotational shaft are engaged with a pair of guides formed in side walls of the image forming device and guided down along the guides. The process unit is accommodated in an accommodating section when both ends of the rotational shaft have been brought into abutment with stops at the ends of the guides and a trailing end of the process unit is rotated downward about the rotational shaft.