Abstract: A semiconductor device capable of measuring a minute current is provided. The semiconductor device includes an operational amplifier and a diode element. An inverting input terminal of the operational amplifier and an input terminal of the diode element are electrically connected to a first terminal to which current is input, and an output terminal of the operational amplifier and an output terminal of the diode element are electrically connected to a second terminal from which voltage is output. A diode-connected transistor that includes a metal oxide in a channel formation region is used as the diode element. Since the off-state current of the transistor is extremely low, a minute current can flow between the first terminal and the second terminal. Thus, when voltage is output from the second terminal, a minute current that flows through the first terminal can be estimated from the voltage.