In an image forming device, a process cartridge is detachably insertable in a process frame. The developing coupling is provided in the one end of the developing unit for transmitting an external rotation force to the developing unit. The developing coupling has: a rotation axis; and a drum shutter to be pivotally moved between a first position at which the drum shutter covers the photosensitive drum and a second position at which the drum shutter does not cover the photosensitive drum. A shutter portion covering the part of the photosensitive drum at the first position. The shutter portion is in direct confrontation with the developing unit at the second position. When the process cartridge is inserted in the process frame, the drum shutter is positioned at the second position whereas when the process cartridge is not inserted in the process frame, the drum shutter is positioned at the first position.