A first output voltage Vcc is obtained via a first switching element from an external power supply, and a second output voltage Vme via a second switching element cascade-connected to the first switching element. The first switching element has a base current continuously controlled by a first comparator/amplifier to maintain the voltage Vcc at a predetermined value. The second switching element has a base current continuously controlled by a second comparator/amplifier to maintain the voltage Vme at a predetermined value. A third switching element connected in parallel to the first switching element and controlled by a duty-factor control circuit makes a bypass power supply to maintain a current flowing through the first switching element not more than a predetermined value. Thus, a constant voltage control device obtaining two types of stabilized voltages from external power supply of large voltage variation can reduce power consumption and improve voltage control accuracy.