A display device is provided including a photosensor PS that detects the ambient light amount of a display area, a comparator that compares the output of the photosensor PS with a predetermined reference value, and a backlight controller that controls the amount of light supplied to the display area depending on a comparison result by the comparator, a switch SW for controlling application of a precharge potential is connected to a metal that exists near the light-receiving part of the photosensor PS with the intermediary of an insulating film. The embodiment apparently decreases the parasitic capacitance of the detection element to thereby suppress the influence of the parasitic capacitance of the detection element itself at the time of light amount detection.