A photosensor includes a lower electrode formed of a metal film, an amorphous silicon film disposed on the lower electrode, an n-type amorphous silicon film disposed on the amorphous silicon film, an upper electrode that is disposed on the n-type amorphous silicon film, and receives a first reference voltage, a capacitive element connected between the lower electrode and a second reference voltage, a switch circuit that inputs a first supply voltage to the lower electrode in an on-state, and puts the lower electrode into a floating state in an off-state, and a detector circuit that detects a voltage change in the lower electrode after irradiating the amorphous silicon film for a given period with light when the switch circuit is on.