The present invention provides a power supply apparatus that automatically determines a driving frequency of an inverter in a system including a plasma generator and the power supply apparatus. A power supply apparatus (10) according to the present invention includes a controller (6) that performs the following action and resonance means (7) for creating a resonance state between the power supply apparatus (10) and a plasma generator (5). That is, the controller (6) changes, when a value of electric power input equal to 100% (or a value of electric power input equal to or more than a threshold % and equal to or less than 100%) of a rated power is instructed, an inverter frequency and obtains an inverter output power factor for each of a plurality of inverter frequencies. Then, the controller (6) determines, as a driving frequency of the inverter, one of the plurality of inverter frequencies at which the inverter output power factor is maximized.