An amplifying unit performs a differential amplification with a highest level or a lowest level of an input signal and a previous input signal. A semiconductor element transfers a signal level output from the amplifying unit from a second terminal to a third terminal by using a current conducted from the second terminal to the third terminal in response to a voltage applied to a first terminal. A control unit controls the voltage applied to the first terminal of the semiconductor element based on a voltage or a current related to a reference semiconductor element. A holding unit holds a signal level output from the third terminal of the semiconductor element.