Vehicle-mounted backup device

A backup device for a vehicle-mounted power source system that includes a main power source configured to supply power to a recipient of power and a power storage device that serves as a power supply source at least when the power supply from the main power source is shut off, the backup device comprising:

• a first conducting path that is provided between the main power source and the recipient of power and to which a voltage based on an output voltage of the main power source is applied when the power supply from the main power source is in a normal condition;
• a second conducting path provided between the power storage device and the first conducting path;
• and
• an element member provided between the first conducting path and the second conducting path configured to:
• (i) restrict a flow of a current from the second conducting path to the first conducting path when the voltage of the second conducting path is smaller than the voltage of the first conducting path, and (ii) allow the flow of a current from the second conducting path to the first conducting path when the voltage of the second conducting path is greater than the voltage of the first conducting path.