Based on a control voltage outputted by a secondary-side control circuit and a drain voltage of a second transistor that has a drain electrode connected to a primary winding of a transformer and performs switching operations based on a gate voltage, a control voltage generating circuit of a synchronous rectifier circuit generates the gate voltage of a first transistor. The gate voltage turns off the first transistor irrespective of the control voltage at timing where the drain voltage falls from a first value to a second value.