An input transistor unit includes a first transistor having a control electrode to which a reference voltage is supplied. An output transistor unit includes a diode-connected second transistor. At least one of the input transistor unit and the output transistor unit further includes a third transistor that is diode-connected and connected in series with the corresponding first transistor or the second transistor and outputs a current in the same direction as the corresponding transistor does. The number of transistors included in the input transistor unit and the number of transistors included in output transistor unit are different from each other. The size of transistors included in the input transistor unit differs from that of transistors included in the output transistor unit.