A semiconductor device having a normal function means is provided, in which the amplitude of an output signal is prevented from being decreased even when a digital circuit using transistors having one conductivity is employed. By turning OFF a diode-connected transistor, the gate terminal of a first transistor is brought into a floating state. At this time, the first transistor is ON and its gate-source voltage is stored in a capacitor. Then, when a potential at the source terminal of the first transistor is increased, a potential at the gate terminal of the first transistor is increased as well by bootstrap effect. As a result, the amplitude of an output signal is prevented from being decreased.