To provide a synchronous rectifying circuit and a power supply unit, capable of enhancing power efficiency, especially under light load, using a simple structure.

This power supply unit 10 is provided with a commutating MOS-FET 16, and a current-detecting resistor 32 is disposed on the upstream side to the MOS-FET 16 in a direction that commutating current I2 flows. During commutation, the energy stored in a chock coil 18 flows via a capacitor 20 and a load 26 to flow commutating current Id2 via the current detecting resistor 32 and an unwanted diode 16A. A comparator 34 compares the reference voltage with a voltage drop by the current detecting resistor 32 and the commutating current I2 and turns on the commutating MOS-FET 16, when the voltage drop is equal to or higher than the reference voltage, thereby flowing a drain current ID2. Only the voltage of an amount of voltage drop by the commutating current I2 is inputted into the comparator 34, so that it is possible to eliminate the need for driving, using another power source.