A developing unit is disclosed which is configured to supply toner to a photosensitive drum in the body of an image forming device, and can be detachably mounted to the body of the image forming device. The developing includes a first developing roller and a roller contacting/retracting mechanism. The first developing roller can be arranged in a position proximate to and parallel with the photosensitive drum, or a position retracted from the photosensitive drum. The roller contacting/retracting mechanism is configured to move the first developing roller to the position proximate to the photosensitive drum when the developing unit is attached to the body of the image forming device, and move the first developing roller to the position retracted from the photosensitive drum when detaching the developing unit from the body of the image forming device.