PURPOSE: To increase reliability in parallel operation by making well-balanced parallel operation of plural switching power supplies to thermally balance them

CONSTITUTION: This switching power supply is provided with an inverter IV which makes on/off operation by a control input and a control means IC for applying the control input to the inverter IV in response to the detection input of an output voltage to a feedback terminal F/B to control the output voltage to be constant, and a difference between its own output voltage and the output voltage of the counterpart is computed on the basis of the parallel operation data of the switching power supply of the counterpart which are inputted from a parallel operation terminal DT by a computing means AMP1, and a computed output is fetched into the feedback terminal F/B of the control means IC. The control means IC outputs a control input for making its own output voltage accord with the output voltage of the switching power supply of the counterpart to the inverter IV on the basis of the computed output to be taken in.