To provide an on-vehicle power supply system capable of preventing a large amount of current from flowing to a chassis even when an ungrounded load is short-circuited.

This on-vehicle power supply system is provided with the chassis for a vehicle and an on-vehicle direct current power supply for full voltage. The full voltage V is divided by m:n to be brought into V1=V×m/(m+n), V2=V×n/(m+ n), and a divided point is grounded commonly to the chassis. Loads 111 and 112 to 11m grounded commonly to the chassis with one end are connected to a system of the partial voltage V1, and loads 121 and 122 to 12n grounded commonly to the chassis with one end are connected to a system of the partial voltage V2. Loads L1 and L2 to Lk not connected to the chassis are connected to a system of the full voltage V. The loads L1 and L2 to Lk are provided with a conductive member to which an electric current is supplied from the on-vehicle power supply, and a non-conductive member electrically isolated from the conductive member. The non-conductive member and the chassis are electrically isolated so that the electric current is not supplied to the chassis when the conductive member and the non-conductive member are short-circuited.