A constant voltage power supply circuit includes an output voltage dependent over-current protection circuit unit and a drooping over-current protection circuit unit. The output voltage dependent over-current protection circuit unit reduces, when an output current which is output from an output terminal of an output control transistor exceeds a first set value which is determined in advance, the output current from the first set value in a manner dependent on a reduction of an output voltage which is output from the output terminal. The drooping over-current protection circuit unit detects, with an external resistor, the output current which is output from the output terminal of the output control transistor, and reduces, when the detected output current exceeds a second set value which is determined in advance and which has a lower value than the first set value, the output voltage while maintaining the output current at the second set value.