An image forming apparatus to detect the toner density of a patch pattern formed on a photosensitive medium or a transfer belt without a deviation in light intensity includes a density sensor including a light emitter to scan light onto at least one patch pattern formed as a toner image and a light receiver to receive light reflected from the at least one patch pattern and convert the received light to an electrical signal, a switch to turn on or off the light emitter, and a controller to maintain light intensity of the light emitter within a reference range by controlling on-time and off-time of the switch.