An object is to suppress deterioration of a high-voltage side battery regardless of the magnitude of a load current. Provided is a control device of a DC-DC converter that is constituted by a primary side circuit that is electrically connected between an input side and a transformer, and a secondary side circuit that is electrically connected between an output side and the transformer. The control device includes a command generating unit 325 that sets an output current limiting value to a predetermined value on the basis of a detected input voltage, a duty generating unit 330 that calculates a duty configured to turn ON/OFF a switching element on the basis of the output current limiting value and a detected output current, and a switching signal generating unit 335 that generates a switching signal on the basis of the duty. The duty generating unit 330 generates the duty so that an output current is limited to the output current limiting value or less.