POWER SUPPLY DEVICE

In a power supply apparatus having an engine operated generator, the operation of the power supply apparatus is improved in the efficiency in a light load region. The generator 1 is driven by the engine. The output of the generator 1 is converted into alternating power of a particular frequency by a rectifier circuit 2 and a converter 4 and is output to a load. A two-way DC-DC converter 5 supplies the power of a battery 6 to the converter 4. When a load current is equal to or less than a predetermined value, the power generated by the battery 6 is supplied to the load, and the power generated by the generator 1 is not used. If the power supply ability of the battery 6 is lowered, the power generated by the generator 1 is supplied to the load, and the battery 6 is charged.