A power supply apparatus for generating a plasma for supplying a high-frequency power to a plasma generating device which is a load. The power supply apparatus comprises: a DC power supply; a power conversion circuit which comprises an amplifier circuit of D-class comprising a plurality of switching elements, and which converts a DC power output of the DC power supply to a high-frequency power to output; and a load impedance conversion circuit which converts a load impedance to a predetermined delayed load, wherein the power supply apparatus is adapted to supply the high-frequency power output from the power conversion circuit to a plasma generating device through the load impedance conversion circuit.