An operational amplifier including a differential input section generating a first signal as a differential voltage between two input signals; an amplifying section amplifying the first signal into second and third complementary signals; a first MOS transistor between a first supply voltage and an output node, a conduction state of the first MOS transistor controlled responsive to the second signal; a second MOS transistor between a second supply voltage and the output node, a conduction state of the second MOS transistor controlled responsive to the third signal; and a step-up section stepping up the first and second supply voltages to generate a step-up voltage higher than the first and second supply voltages, the amplifying section driven by the step-up voltage so that an absolute value of the maximum level of the second or third signal becomes larger than the absolute value of the first or second supply voltage.