A constant-voltage power circuit incorporating an over-current protection circuit having a fold back current limiting capability operative through low input voltages, and being capable of arbitrarily setting current outputs. The constant-voltage power supply circuit is configured so that a voltage output with small amplitude from a differential amplifier circuit is subjected to amplitude expansion to be amplified to range fully from the ground potential approximately to the input voltage by means of an amplitude expansion circuit, which includes an inverter consisting of an NMOS transistor and a resistor, and that the voltage resulting from the amplitude expansion is subsequently input to the gate of a PMOS transistor configured to directly control an output transistor.