In conventional multi-output switching power supply device, power is supplied from a relatively high voltage side output to a relatively low voltage side output through a dropper circuit which generates relatively large power loss so as to improve the voltage accuracy of a non-stabilized output, so that power supply efficiency is low and heat generated from the dropper circuit is high.One DC power supply of a plurality of DC power supplies on the secondary side is a stabilized output (24 V output terminal TM3) having a voltage stabilizing means for stabilizing the output voltage by feeding back the output voltage to a primary side control circuit 4, and the rest of the DC power supplies are non-stabilized outputs (12 V output terminals TM4) not having a voltage stabilizing means for feeding back the output voltage to the primary side. A step-up/down converter is provided between the output of the non-stabilized output and the output of the stabilized output and power is applied/received between the outputs in accordance with the non-stabilized output voltage and, thus, the voltage accuracy of the non-stabilized output is improved.