To realize a highly efficient forward converter.

In a secondary output circuit constituting a forward converter, connecting in parallel with both ends of a rectifier diode 2 a bypass circuit comprising a switching element 4 and a capacitor 5, a controlling voltage of the secondary output circuit is fed to a control circuit 9 via the capacitor 5 to subject alternately a switching element 1 provided in the primary side of the forward converter and the switching element 4 of the bypass circuit to ON-OFF controls. As a result, not only the breakdown voltage of the primary- side switching element 1 can be made allowably low, but also the utilization factor of the transformer of the forward converter can be improved. Also, in the forward converter of a synchronous rectification mode, since the driving waveforms of its switching elements are made rectangular and improved, its high efficiency can be realized.