A power source for a hybrid vehicle is provided which can protect its electric motor driving circuit from being damaged by a back electromotive force generated in a motor. The back electromotive force generated in an electric motor is detected by a high voltage detector circuit and converted by a step-down circuit into a low voltage which is then stored in a low-voltage second battery. Accordingly, as the back electromotive force does not become excessively high, it will be prevented from damaging a switching circuit. Also, the back electromotive force generated in the motor is charged in the low-voltage battery and can thus be supplied to a control circuit or any other component, contributing to the reduction of the fuel consumption of the hybrid vehicle. The back electromotive force can alternatively be diverted for charging a medium-voltage third battery.