A housing assembly of an OPC unit to protect an OPC drum in an image forming apparatus includes a main housing enclosing at least a half of a large-diameter OPC drum and provided with a main-opening selectively contacting a plurality of developing units and a plurality of sub-openings, a first housing and a second housing each mounted on a side of the main housing, a handle hingedly connected to the main housing, and a plurality of brackets mounted on opposite sides of the main housing. As a result, the housing assembly of the OPC unit allows easy development, electrification, exposure, image transfer and treatment of a waste toner, and reliability of the image developing process increases due to the fixation of the developing units. Also, since the exposure of the OPC drum is minimized, an area of the OPC drum to be contaminated can be reduced.