A plurality of first sensor pixel circuits each sensing light during a sensing period when a backlight is turned on and retaining the amount of sensed light otherwise in accordance with a clock signal and a plurality of second sensor pixel circuits each sensing light during a sensing period when the backlight is turned off and retaining the amount of sensed light otherwise in accordance with a clock signal are arranged in a pixel region. The sensor pixel circuits of two types are connected to different output lines, so that a difference between two output signals is obtained at the outside of the sensor pixel circuit. The sensor pixel circuits described above are used for detecting a difference between an amount of light to be incident when the backlight is turned on and an amount of light to be incident when the backlight is turned off.