To prevent the burning and smoke generation of control transistors because a protection circuit is controlled as inoperative.

In a power supply and a power feeding method, a voltage or current supplied to a load is maintained to a constant value with a predetermined integrated circuit 1 without relation to the variation of an input voltage, and the supply voltage or current is detected with a detector 3. When a microcomputer 4 has determined that the detected value is within a predetermined range, a signal to stop an output is outputted to the integrated circuit. For the integrated circuit, a circuit forming a voltage-drop type, for example, a DC/DC converter which outputs a constant voltage with a switching method is used.