An adverse effect of parasitic capacitance on optical data output from a photodetector circuit is suppressed. A photodetector circuit includes a photoelectric conversion element; a first field-effect transistor; a second field-effect transistor; a first conductive layer functioning as a gate of the first field-effect transistor; an insulating layer provided over the first conductive layer; a semiconductor layer overlapping with the first conductive layer with the insulating layer interposed therebetween; a second conductive layer electrically connected to the semiconductor layer; and a third conductive layer electrically connected to the semiconductor layer, whose pair of side surfaces facing each other overlaps with at least one conductive layer including the first conductive layer with the insulating layer interposed therebetween, and which functions as the other of the source and the drain of the first field-effect transistor.