A plurality of first and second sensor pixel circuits each sensing light during a designated sensing period and retaining the amount of sensed light otherwise are arranged in a pixel region. A backlight is turned on once for a predetermined time in one-frame period. A sensing period when the backlight is turned on and a sensing period when the backlight is turned off are set once, respectively, in the one-frame period. The first sensor pixel circuit is reset. The second sensor pixel circuit is reset. Read from sensor pixel circuits of two types is performed in parallel in a line sequential manner during a period other than the periods and. A difference circuit provided outside of the sensor pixel circuits is used for obtaining a difference between an amount of light when the backlight is turned on and an amount of light when the backlight is turned off.