To provide an overcurrent protection circuit, which can set a temperature coefficient of short-circuit current to 0 or less while reducing the circuit scale, and a constant-voltage power supply circuit.

The overcurrent protection circuit includes an overcurrent limiting part which controls an output transistor so that output current never exceeds an upper limit value; a current voltage conversion circuit which inputs a part of the output current according to reduction in output voltage of the output transistor and outputs it while converting to voltage; a short-circuit current limiting part which controls the output transistor by adjusting the output of the current voltage conversion circuit to reduce the output current according to the reduction in output voltage. The output of the current voltage conversion circuit and the transformer impedance of the current voltage conversion circuit are set so that the output current is reduced relative to rise of temperature when the output voltage is 0.