PURPOSE: To supply power with high efficiency and high accuracy from a low output to a high output by changing over the step-down means of DC input voltage between a chopper step-down means and a dropper step-down means in response to a signal from a current detecting means.

CONSTITUTION: A resistor R12 is installed between an input terminal 2 and an output terminal 4, voltage-divider resistors R1, R2, R3 are combined, and voltage V1 and V2 for detecting output currents il is obtained. A changeover switch SW is brought to the state of a-c connection by an output from a comparator CP2 when voltage V2 is lower than reference voltage Vref2, when output currents 11 is a fixed value or less, and the output from the comparator CP2 is changed and the changeover switch is changed over to the state of a-b connection when output currents 11 exceed the fixed value and voltage V2 is made higher than reference voltage Vref2. Accordingly, a transistor Q1 is controlled through a drop system when output currents 11 are reduced and through a step-down chopper system when output currents are increased.