PURPOSE: To obtain a switching power source in which synchronous rectification with low loss can be conducted by detecting a current flowing to a rectifying switching element and turning ON, OFF a synchronously rectifying switching element.

CONSTITUTION: When a current flows to a current transformer 14 through a body diode 11 of a second switching element 6, a current inversely proportional to the number of turns of an output winding n6 of the transformer 14 flows in the winding n6. This current flows to a resistor 16 through a diode 15 to generate a voltage, which is input to a (+) terminal of a comparator 18. As a result, the output of the comparator 18 reaches an H level to turn ON the element 6 through a second switching element driving circuit 12. Energy stored in a primary winding n1 of a transformer 5 is converted via a secondary winding n2, and fed to the element 6, and when the current becomes zero, potentials of the (+) terminal and (-) terminal of the comparator 18 are inverted, and the output of the comparator 18 becomes an L level, thereby turning OFF the element 6 through the circuit 12.