PURPOSE: To reduce the size and cost of an electric power source by connecting plural boosting transformers to a common switching element in parallel and driving them.

CONSTITUTION: A driving signal is supplied from a driving circuit 16 to the switching element Q, which performs switching operation at a specific frequency to develop AC voltages across primary windings N21 and N22 of the boosting transformers T1 and T2. Consequently, AC voltages are induced across secondary windings N21 and N22 and supplied to a load through rectifying circuits respectively. The current flowing through the load RL2 is detected by an output current detecting element R5 and compared by a comparator AS with a reference voltage Vref to send their deviation voltage to the driving circuit 16. Then, the driving circuit 16 sends a driving signal with pulse width corresponding to the deviation voltage to the switching element Q.