A controller of a DC-DC converter is configured to: obtain respective temperatures of a plurality of switching elements from a plurality of temperature sensors while the controller is driving all of the plurality of switching elements; determine at least one preservation target switching element from the plurality of switching elements based on the respective temperatures of the plurality of switching elements such that the at least one preservation target switching element includes a switching element a temperature of which is highest among the plurality of switching elements; and drive at least one non-preservation target switching element other than the at least one preservation target switching element to adjust an output current of a DC-DC converting circuit to a target value.