An object is to provide a semiconductor device which solves a problem that can occur when a substrate having an insulating surface is used. The semiconductor device includes a base substrate having an insulating surface; a conductive layer over the insulating surface; an insulating layer over the conductive layer; a semiconductor layer having a channel formation region, a first impurity region, a second impurity region, and a third impurity region provided between the channel formation region and the second impurity region over the insulating layer; a gate insulating layer configured to cover the semiconductor layer; a gate electrode over the gate insulating layer; a first electrode electrically connected to the first impurity region; and a second electrode electrically connected to the second impurity region. The conductive layer is held at a given potential.