A reference voltage generating circuit in a DC power supply includes a differential amplifier for detecting a difference voltage between a reference voltage and the output voltage, and a voltage control circuit which is responsive to an output signal from the differential amplifier so as to control the DC power supply output voltage to limit the difference between the output voltage and the reference voltage to zero. More specifically, the reference voltage generating circuit includes a reference voltage source, a first variable resistor connected in parallel with the reference voltage source, a second variable resistor connected to the negative side of the reference voltage source, a third variable resistor connected to the positive side of the reference voltage source, and a switching circuit for connecting a movable contact piece of the first variable resistor to either the second or third variable resistors, or to neither of them. A predetermined voltage of a magnitude greater than, equal to or less than a rated value can be delivered as a reference voltage at the movable contact piece of the first variable resistor by selecting from a plurality of preadjusted voltage divider circuits using a single selection operation.