A voltage regulator circuit includes a transistor and a capacitor. The transistor includes a gate, a source, and a drain, a first signal is inputted to one of the source and the drain, a second signal which is a clock signal is inputted to the gate, an oxide semiconductor layer is used for a channel formation layer, and an off-state current is less than or equal to 10 aA/μm. The capacitor includes a first electrode and a second electrode, the first electrode is electrically connected to the other of the source and the drain of the transistor, and a high power source voltage and a low power source voltage are alternately applied to the second electrode.