A control circuit controlling the driving of a switching device includes an under voltage lock out circuit stopping the operation of the control circuit when a power supply voltage supplied from the auxiliary winding of an insulating transformer becomes lower than a predetermined power supply lower limit voltage, an FB voltage decision circuit stopping the driving of the switching device when a feedback voltage indicating the state of a load becomes lower than the predetermined threshold value, a flip-flop brought into a set state by an output of the FB voltage decision circuit and a reset state when the power supply voltage exceeds the voltage ensuring the normal operation of the control circuit, and a switching circuit to set the power supply lower limit voltage in the under voltage lock out circuit, lower than that of the normal operation when the flip-flop is set.