A power supply system that is configured to be provided between a commercial power system and an important load and is configured to supply electric power to the important load, wherein the important load is a load to which electric power is to be stably supplied even at a time of a commercial power system abnormality, the power supply system, characterized in that, it comprises: a distributed power supply that is configured to be connected to a power line, the power line is configured to supply electric power from the commercial power system to the important load; a changeover switch that is configured to open and close the power line; an impedance element that is configured to be connected in parallel to the changeover switch in the power line; a system-side voltage detecting unit that is configured to be located between the changeover switch and the commercial power system and is configured to detect a voltage on the commercial power system regardless of opening or closing of the changeover switch; and a control unit that is configured to open the changeover switch and is configured to connect the distributed power supply and the commercial power system via the impedance element when a voltage detected by the system-side voltage detecting unit becomes equal to or lower than a predetermined set value, wherein the distributed power supply is configured to continue to perform a power generation and a reverse power flow in a state in which the distributed power supply and the commercial power system are connected via the impedance element, wherein the power supply system further comprises a frequency variation detecting unit that is configured to be located between the changeover switch and the commercial power system and is configured to detect a frequency variation from the voltage detected by the system-side voltage detecting unit, and wherein, when a frequency tolerance of the important load or the distributed power supply is not within a predetermined set range and the frequency variation detected by the frequency variation detecting unit is equal to or greater than the frequency tolerance and is included in the predetermined set range, the control unit is configured to open the changeover switch and is configured to connect the distributed power supply and the commercial power system via the impedance element. a distributed power supply that is configured to be connected to a power line, the power line is configured to supply electric power from the commercial power system to the important load; a changeover switch that is configured to open and close the power line; an impedance element that is configured to be connected in parallel to the changeover switch in the power line; a system-side voltage detecting unit that is configured to be located between the changeover switch and the commercial power system and is configured to detect a voltage on the commercial power system regardless of opening or closing of the changeover switch; and a control unit that is configured to open the changeover switch and is configured to connect the distributed power supply and the commercial power system via the impedance element when a voltage detected by the system-side voltage detecting unit becomes equal to or lower than a predetermined set value, wherein the distributed power supply is configured to continue to perform a power generation and a reverse power flow in a state in which the distributed power supply and the commercial power system are connected via the impedance element, wherein the power supply system further comprises a frequency variation detecting unit that is configured to be located between the changeover switch and the commercial power system and is configured to detect a frequency variation from the voltage detected by the system-side voltage detecting unit, and wherein, when a frequency tolerance of the important load or the distributed power supply is not within a predetermined set range and the frequency variation detected by the frequency variation detecting unit is equal to or greater than the frequency tolerance and is included in the predetermined set range, the control unit is configured to open the changeover switch and is configured to connect the distributed power supply and the commercial power system via the impedance element.