An electronic control unit (40) calculates the current flowing through a reactor (L) by dividing an output required BP* of a battery (32), obtained from converting the power required by a motor (22), by a terminal voltage Vb of the battery (32). A carrier frequency (optimum carrier frequency) is set for transistors (T7, T8) where the loss of a DC/DC converter (34) is minimized from the calculated current, and the DC/DC converter (34) is controlled at the set switching frequency.