To provide a protection circuit having a small area, redundancy, and small leak current. In the protection circuit, a plurality of nonlinear elements is provided so as to overlap with each other and so as to be electrically connected in series. At least one nonlinear element in the protection circuit is a diode-connected transistor including an oxide semiconductor in its channel formation region. The other nonlinear element is a diode-connected transistor including silicon in its channel formation region or a diode including silicon in its junction region.