To provide a power conversion device, which enables supplying power of high quality without blackout to apparatuses such as information processing devices, or the like, for which stabilization of power supply is always required; which effectively utilizes as much as possible energies obtainable from DC power supplies such as fuel cells, secondary batteries, solar cells, wind power generation means, or the like; and further whose circuit structure to control those is simple; and an electric power supply system using it.

In the power conversion device 1, comprising a first AC conversion part 4 which has a first bridge circuit 3 comprising a plurality of semiconductor switching elements; and a second AC conversion part 6 which has a second bridge circuit 5 comprising a plurality of semiconductor switching elements, the AC power converted in the first Ac conversion part 4 is subjected to grid-connected operation with a commercial power supply system, and the AC power converted in the second AC conversion part 6 is outputted to an exclusive load 8 so that a stable power source supply to the exclusive load 8 can be executed independent of the condition of the commercial power supply system.