An electric power supply system is provided for feeding an output of a DC power source to a load at high efficiency and, without complicated controlling, for allowing for interchange of electric power among a commercial power system, a DC power source and a load.An electric power supply system includes a DC bus line for connecting a DC generator, a load and system power. The electric power supply system comprises a DC/DC converter connected between the DC generator and the DC bus line for controlling an output voltage to the DC bus line at a voltage V1; an inverter connected between the system power and the DC bus line for operating a control in response to an input voltage V2 from the DC bus line; and an AC/DC converter connected between the system power and the DC bus line for controlling an output voltage to the DC bus line at a voltage V3, wherein the respective voltages are set as V1>V2>V3.