An insulated power supply circuit includes a power input circuit including a switching control circuit and a switching element connected to a corresponding winding of a transformer, and power output circuits of two systems each including a regulator connected to a corresponding winding. By the control circuit controlling ON/OFF of the switching element in accordance with an output condition change command signal, which is generated based on a load condition at an output destination of each regulator, a voltage corresponding to an estimated value of a preset excitation level is generated, to thereby change the excitation level of the winding. Each regulator receives an output voltage generated in a corresponding winding in response to the change in excitation level.