In an amplifier that uses a transistor, a minimum operation voltage is lowered.An amplifier includes a P-type transistor and an N-type transistor connected in series, and an operational amplifier. An output terminal of the operational amplifier is connected to gates of both the P-type transistor and the N-type transistor. One of an inverting input terminal and a non-inverting input terminal of the operational amplifier is connected to drains of both the P-type transistor and the N-type transistor. Further, a predetermined reference voltage is applied to another of the inverting input terminal and the non-inverting input terminal.