A switching control IC is provided with a circuit for generating a triangular wave using a frequency that is dependent on a capacitor connected to a CT terminal of the switching control IC, and a resistor connected to an RT terminal thereof. A synchronizing signal input circuit for externally receiving a synchronizing signal that is a pulse voltage signal is connected to the CT terminal. A charging voltage for the capacitor connected to the CT terminal is changed by the synchronizing signal, which synchronizes the frequency of the triangular wave with the synchronizing signal, whereby the frequencies of a plurality of DC-DC converters are synchronized.