To stably control a threshold voltage of a functional circuit using an oxide semiconductor. A variable bias circuit, a monitoring oxide semiconductor transistor including a back gate, a current source, a differential amplifier, a reference voltage source, and a functional circuit which includes an oxide semiconductor transistor including a back gate are provided. The current source supplies current between a source and a drain of the monitoring oxide semiconductor transistor to generate a gate-source voltage in accordance with the current. The differential amplifier compares the voltage with a voltage of the reference voltage source, amplifies a difference, and outputs a resulting voltage to the variable bias circuit. The variable bias circuit is controlled by an output of the differential amplifier and supplies voltage to the back gate of the monitoring oxide semiconductor transistor and the back gate of the oxide semiconductor transistor included in the functional circuit.