A power supply apparatus activating method, in which the power supply apparatus includes a first line, a second line, and a third line having different potentials which sequentially decrease; a battery circuit where a first power supply and a second power supply are connected in series to each other; and a DC-DC converter formed by a switching circuit where a first switching element and a second switching element respectively having free-wheeling diodes are connected in series to each other, and a reactor, and in which two ends of the battery circuit are connected to the first line and the third line, a junction between the first power supply and the second power supply of the battery circuit is connected to the second line, an end portion on the first switching element side of the switching circuit is connected to the first line, an end portion on the second switching element side of the switching circuit is connected to the third line, one end of the reactor is connected to a junction between the first and second switching elements, and the other end of the reactor is connected to the second line, wherein the method is characterized in that it comprises: a step of executing a one-side switching mode in which, when the power supply apparatus is activated, while an ON state of the first switching element is inhibited, only the second switching element is alternately turned ON and OFF, with the ON state duration of the second switching element changed so as to become longer; and a step of executing an alternating switching mode in which the first switching element and the second switching element are alternately turned ON, wherein the one-side switching mode is executed before the alternating switching mode, wherein, when the one-side switching mode is executed, in a case where a current continuously flows through the reactor within a switching cycle of the second switching element, the execution of the one-side switching mode is switched to the execution of the alternating switching mode, and wherein, during one switching cycle of the second switching element when the one-side switching mode is executed, in a case where a terminal end of the current of a triangular waveform flowing through the reactor matches an end of the switching cycle, the execution of the one-side switching mode is switched to the execution of the alternating switching mode. a step of executing a one-side switching mode in which, when the power supply apparatus is activated, while an ON state of the first switching element is inhibited, only the second switching element is alternately turned ON and OFF, with the ON state duration of the second switching element changed so as to become longer; and a step of executing an alternating switching mode in which the first switching element and the second switching element are alternately turned ON, wherein the one-side switching mode is executed before the alternating switching mode, wherein, when the one-side switching mode is executed, in a case where a current continuously flows through the reactor within a switching cycle of the second switching element, the execution of the one-side switching mode is switched to the execution of the alternating switching mode, and wherein, during one switching cycle of the second switching element when the one-side switching mode is executed, in a case where a terminal end of the current of a triangular waveform flowing through the reactor matches an end of the switching cycle, the execution of the one-side switching mode is switched to the execution of the alternating switching mode.