A dual-gate transistor including: a first insulating layer provided to cover a first conductive layer; a first semiconductor layer over the first insulating layer; second semiconductor layers over the first semiconductor layer, the second semiconductor layers are spaced from each other to expose the first semiconductor layer; impurity semiconductor layers over the second semiconductor layers; second conductive layers over the impurity semiconductor layers; second insulating layers over the second conductive layers; a third insulating layer to cover the first semiconductor layer, the second semiconductor layers, the impurity semiconductor layers, the second conductive layers, and the second insulating layers; and a third conductive layer at least over the third insulating layer, and in the dual-gate transistor including the first to third insulating layers with openings, the first insulating layer is substantially equal in thickness to the second insulating layer.