To reduce parasitic capacitance between a gate electrode and a source electrode or drain electrode of a dual-gate transistor. A semiconductor device includes a first insulating layer covering a first conductive layer; a first semiconductor layer, second semiconductor layers, and an impurity semiconductor layer sequentially provided over the first insulating layer; a second conductive layer over and at least partially in contact with the impurity semiconductor layer; a second insulating layer over the second conductive layer; a third insulating layer covering the three semiconductor layers, the second conductive layer, and the second insulating layer; and a third conductive layer over the third insulating layer. The third conductive layer overlaps with a portion of the first semiconductor layer, which does not overlap with the second semiconductor layers, and further overlaps with part of the second conductive layer.