An isolated switching power supply can include a switching element connected to an input power supply, a control circuit that ON/OFF-drives the switching element and generates an AC power in a secondary winding and in an auxiliary winding of the isolation transformer, an output circuit that rectifies the AC power generated in the on a secondary winding of the isolation transformer and outputs the rectified power, an internal power supply circuit that generates a driving power supply voltage for the control circuit from the AC power generated in the auxiliary winding, an output voltage detecting circuit that feeds back an output monitoring voltage obtained from the driving power supply voltage to the control circuit to control the ON/OFF driving of the switching element by the control circuit and an output voltage controller that changes a level of the output monitoring voltage obtained from the driving power supply voltage.