A current mirror circuit converts a positive-side supply voltage of a second power supply with respect to a reference potential into an electric current so as to supply the electric current to a current/voltage converting section. The current/voltage converting section converts the electric current from the current mirror circuit into a voltage with respect to the reference potential of a first power supply so as to supply the voltage to a feedback input terminal of a switching controller. Therefore, although the reference potential of the first power supply supplied to the switching controller is different from the reference potential of the second power supply, the electric current from the current mirror circuit is not influenced by the reference potential.