A power supply system includes; a first switch connected between a main power source and loads; a second switch connected between a sub-battery and the loads; and backflow prevention circuit that prevent current backflow between the main power source and the sub-battery, and polarity of the backflow prevention circuit is devised so as to allow current passage in a direction from the sub-power source to the loads even when the second switch is in an off state. When power from the main power source is interrupted, the power source power of the sub-power source is supplied to the loads via the backflow prevention circuit even the second switch remains in the off state, so that the power supply is not interrupted.