A photosensitive belt installation device of an electrophotographic image forming apparatus that includes a soft cartridge which rolls up for storage, and an installation guide member which selectively attaches to the image forming apparatus. When unrolled, the cartridge, allows the simple installation of a photosensitive belt into the imaging forming apparatus. The soft cartridge is a flexible cartridge having a shape of the endless track of a belt unit of the imaging forming apparatus, and is open on both sides. The unrolled soft cartridge supports the photosensitive belt during installation, as one open side of the soft cartridge is fitted over a sloping surface of the installation guide member attached to the front frame of the belt unit. The soft cartridge is completely pushed around the belt unit along a sloping surface of the installation guide member. The photosensitive belt remains around the belt unit as the soft cartridge is pulled out. The installation guide member is then be removed from the front of the belt unit.