A power source apparatus for supplying power to a load, being a class E amplifier and comprising: a switch; a controller (30, 30A, 30A', 30B, 30C, 30D, 30E) that: is configured to detect a voltage applied to the switch, and is adapted to control a timing of switching a state of the switch from OFF to ON when the detected voltage is equal to or smaller than a predetermined threshold value, a first DC power source that is configured to apply a DC voltage between terminals of the switch, and a compensating circuit (42, 42F, 42G, 42H) that is connected in parallel to the switch and that comprises a second DC power source and a capacitor, wherein the controller is adapted to control a combined capacitance of parasitic capacitances of the switch and the compensating circuit to be constant by changing a voltage of the second DC power source, and wherein the power source apparatus further comprises: a thermistor that is configured to detect a temperature of the switch, and a memory that is configured to store either a table associating the temperature with the voltage of the second DC power source or a prescribed formula in which the temperature and the voltage of the second DC power source result in the combined capacitance having a fixed value, wherein the controller is adapted to control the voltage of the second DC power source based on the table or the prescribed formula so that the combined capacitance becomes constant. a switch; a controller (30, 30A, 30A', 30B, 30C, 30D, 30E) that: is configured to detect a voltage applied to the switch, and is adapted to control a timing of switching a state of the switch from OFF to ON when the detected voltage is equal to or smaller than a predetermined threshold value, a first DC power source that is configured to apply a DC voltage between terminals of the switch, and a compensating circuit (42, 42F, 42G, 42H) that is connected in parallel to the switch and that comprises a second DC power source and a capacitor, wherein the controller is adapted to control a combined capacitance of parasitic capacitances of the switch and the compensating circuit to be constant by changing a voltage of the second DC power source, and wherein the power source apparatus further comprises: a thermistor that is configured to detect a temperature of the switch, and a memory that is configured to store either a table associating the temperature with the voltage of the second DC power source or a prescribed formula in which the temperature and the voltage of the second DC power source result in the combined capacitance having a fixed value, wherein the controller is adapted to control the voltage of the second DC power source based on the table or the prescribed formula so that the combined capacitance becomes constant. a thermistor that is configured to detect a temperature of the switch, and a memory that is configured to store either a table associating the temperature with the voltage of the second DC power source or a prescribed formula in which the temperature and the voltage of the second DC power source result in the combined capacitance having a fixed value, wherein the controller is adapted to control the voltage of the second DC power source based on the table or the prescribed formula so that the combined capacitance becomes constant.