A constant voltage circuit configured to convert an input voltage into an output voltage having a predetermined level is disclosed. The constant voltage circuit includes a differential amplifier circuit configured to produce an output signal having a voltage level in response to a reference voltage and the output voltage; and an output circuit configured to receive the output signal and produce a current in response to the voltage level of the output signal. The output voltage is proportional to the current. The output circuit includes plural output transistors and a transistor selecting unit configured to select one or more output transistors to be operated among the plural output transistors to produce the current depending on the level of the output voltage.