A CMOS inverter capable of operating at low voltages is provided. The gate of a p-channel MOS transistor and the gate of an n-channel MOS transistor are AC coupled to an input terminal via first and second capacitors, respectively. Signals whose amplitude centers are optimized according to the threshold voltages of the p- and n-channel MOS transistors by bias voltages from first and second variable voltage sources, respectively, are supplied to the gates of these MOS transistors. In consequence, the CMOS inverter can operate at high speeds at low power supply voltages without being affected by the threshold voltages.