The present invention aims to obtain a stable output voltage without requiring an extra circuit by suppressing influence of noise of the primary winding and voltage induction by leakage inductance. In a transformer 3 used in a multi-output switching power supply, a secondary winding S1 for outputting a voltage to be fed back to the primary side out of the secondary windings S2 to Sn winded on a bobbin 3a is divided into a first layer S1a and a second layer S1b, a secondary winding S2 to Sn for outputting a voltage not fed back to the primary side is winded between the first layer S1a and the second layer S1b, and the first layer S1a and the second layer S1b is regular winded.