A boost DC/DC includes an inductor connected at one end to the positive-pole terminal of a low-voltage-side port, a transformer of a magnetic-field cancellation type including a primary winding and a secondary winding interconnected in an oppositely-wound configuration, a common terminal of the primary winding and the secondary winding being connected to another end of the inductor. A switching element controls an energizing current of the primary winding flowing to a common reference terminal, and another switching element controls an energizing current of the secondary winding flowing to the common reference terminal. A diode allows current to flow in only one direction from the primary winding to the positive-pole terminal of a high-voltage-side port, and another diode allows current to flow in only one direction from the secondary winding to the positive-pole terminal of the high-voltage-side port.