To provide a switching power supply with reduced common mode noise, by improving a winding structure of a transformer.

A primary input through a switching operation is fed to a primary winding N1, and a secondary output according to the switching operation is obtained by using the primary winding N1 and a secondary winding N2, which is insulated from the primary winding N1. A switching power supply has an auxiliary primary winding N3 having the same turns and the same quantity of field as the primary winding N1 and put in parallel for generating opposite polarity to the primary winding N1. The secondary winding N2 is put between the primary winding N1 and the auxiliary winding N3. By an off-set effect of these reverse field of the primary and auxiliary windings N1 and N3, an influence of field from the primary side to the secondary side of a transformer T1 can be canceled.