Abstract: A power supply device produces a predetermined voltage from a supply voltage through a voltage follower composed of a plurality of transistors. The voltage follower has a function of preventing backflow current. The voltage follower has one end connected to the supply voltage and another end connected to a capacitor. A constant voltage generating circuit produces a constant voltage from the voltage across the capacitor. A switch circuit is connected between the supply voltage and the control electrode of one of the transistors constituting the voltage follower other than the first-stage transistor thereof. A voltage drop detecting circuit detects a drop below a predetermined level in the voltage across the capacitor, and turns on the switch circuit on detecting such a drop.