A power supply apparatus arranged to output an alternating-current voltage to a plasma generator arranged to generate an ozone gas and a radical gas, the plasma generator being a capacitive load including a plurality of discharge cells connected to one another, said power supply apparatus comprising: an inverter arranged to convert direct-current electric power to alternating-current electric power; a controller arranged to control an action of said inverter; a detection unit (31, 32) that is provided for said inverter and is arranged to detect an electricity amount used to obtain an inverter output power factor; and resonance means arranged to create a resonance state between said power supply apparatus and said plasma generator, wherein said controller is arranged to change, when an instructed value of electric power input is equal to or more than a threshold % and is equal to or less than 100% of a rated power of said power supply apparatus, an inverter frequency being a control value for said inverter and obtain said inverter output power factor for each of a plurality of inverter frequencies on the basis of said electricity amount acquired from said detection unit, and determine, as a driving frequency of said inverter, one of said plurality of inverter frequencies at which said inverter output power factor obtained by the controller is maximized, characterized in that said controller is arranged to fixedly output, after determining, as a driving frequency of the inverter, one of said plurality of inverter frequencies at which said inverter output power factor obtained by the controller is maximized, said driving frequency to said inverter when an instructed value of the electric power input that is less than said threshold % of the rated power of said power supply apparatus is instructed. an inverter arranged to convert direct-current electric power to alternating-current electric power; a controller arranged to control an action of said inverter; a detection unit (31, 32) that is provided for said inverter and is arranged to detect an electricity amount used to obtain an inverter output power factor; and resonance means arranged to create a resonance state between said power supply apparatus and said plasma generator, wherein said controller is arranged to change, when an instructed value of electric power input is equal to or more than a threshold % and is equal to or less than 100% of a rated power of said power supply apparatus, an inverter frequency being a control value for said inverter and obtain said inverter output power factor for each of a plurality of inverter frequencies on the basis of said electricity amount acquired from said detection unit, and determine, as a driving frequency of said inverter, one of said plurality of inverter frequencies at which said inverter output power factor obtained by the controller is maximized, characterized in that said controller is arranged to fixedly output, after determining, as a driving frequency of the inverter, one of said plurality of inverter frequencies at which said inverter output power factor obtained by the controller is maximized, said driving frequency to said inverter when an instructed value of the electric power input that is less than said threshold % of the rated power of said power supply apparatus is instructed. change, when an instructed value of electric power input is equal to or more than a threshold % and is equal to or less than 100% of a rated power of said power supply apparatus, an inverter frequency being a control value for said inverter and obtain said inverter output power factor for each of a plurality of inverter frequencies on the basis of said electricity amount acquired from said detection unit, and determine, as a driving frequency of said inverter, one of said plurality of inverter frequencies at which said inverter output power factor obtained by the controller is maximized, characterized in that said controller is arranged to fixedly output, after determining, as a driving frequency of the inverter, one of said plurality of inverter frequencies at which said inverter output power factor obtained by the controller is maximized, said driving frequency to said inverter when an instructed value of the electric power input that is less than said threshold % of the rated power of said power supply apparatus is instructed.