A reliable power conditioner in a system having three power sources: a direct-current power source such as a solar cell, an accumulating unit such as an accumulator, and a commercial power system.The power conditioner is provided: a power converting circuit for converting direct-current power obtained from a direct-current power source to alternating-current power of a commercial power system; a charger/discharger circuit for charging the direct-current power obtained from the direct-current power source to an accumulator or discharging the direct-current power stored in the accumulator; a control circuit for controlling the power converting circuit and the charger/discharger circuit; a first power source circuit for supplying power from the direct-current power source to the control circuit; a second power source circuit for supplying power from the accumulator to the control circuit; a third power source circuit for supplying power from the commercial power system to the control circuit; and a power source selecting circuit for selecting at least one of the first, second, and third power source circuits and supplying power to the control circuit.