1. An image forming apparatus comprising: a plurality of photosensitive drums; an exposing unit to irradiate light onto each of the photosensitive drums; a plurality of developing units, each respective developing unit to form a toner image by developing an electrostatic latent image formed on the respective photosensitive drum; a transfer member; at least one registration sensor including a light emitter to irradiate light in a direction toward the transfer member and a light receiver to receive reflected light and to output a signal corresponding to an amount of the reflected light; and a processor to determine image alignment error based on signals output by the light receiver of the at least one registration sensor; wherein the transfer member and the at least one registration sensor are incorporated within a single transfer unit such that when the transfer unit is replaced, the transfer member and the at least one registration sensor are simultaneously replaced.