To provide a switching power supply that suppresses loss.

The primary winding 2A of a transformer 1A is connected in series to the primary winding 2B of a transformer 1B. At the secondary sides of the transformers 1A, 1B, secondary side circuits each consisting of diodes 20A, 21A and a choke coil 22A and of diodes 20B, 21B and a choke coil 22B are provided at the rear stage of the secondary windings 3A, 3B respectively and these secondary side circuits are connected in parallel. By dividing transformers into plural units and by connecting two primary windings 2A, 2B in series, it is possible to halve winding wire ratio for a voltage supplied to the primary windings to become a half. Thus, leakage inductance can be made small and efficiency can be improved.