An imaging forming apparatus includes a platen on which an original is put, and a platen cover mounted to be opened or closed with respect to the platen. A plurality of reflection type photosensors are embedded in the platen cover at predetermined positions. First, a sensor circuit including photosensors is set in a first mode. When an original is put on the platen and the platen cover is closed, photosensors detect the reflection light from the original such that the CPU can determine a size of the original based upon sensor inputs from the sensor circuit. Thereafter, in order to detect opening or closing of the platen cover, the sensor circuit is set in a second mode. In the second mode, total sensitivity of a photosensor for detecting a minimum size of the original is heightened, and light emitting diodes of all photosensors are inhibited from being driven such that the photosensors can respond to only external light. In the second mode, when the platen cover is closed, since the external light that enters the photosensors becomes approximately zero, no detection signal is outputted from even the photosensor having higher total sensitivity, and therefore, it is possible to detect that the platen cover is closed.