A secondary-side rectifier circuit includes an adder-rectifier circuit (80) that stores a voltage generated in a secondary winding in a capacitor as electrostatic energy in an on period of one of a high-side and low-side switching circuits (Q1) or (Q2), and adds the voltage in the capacitor and the voltage generated in the secondary winding and outputs the sum as a direct-current voltage during in an on period of the other of the high-side and low-side switching circuits (Q2) or (Q1). A switching control circuit (10) adjusts an output power to be output from the secondary-side rectifier circuit, by using on-period ratio control means for controlling a proportion of periods during which the respective high-side side and low-side switching elements (Q1, Q2) are brought into a conductive state. Accordingly, a switching power supply device with reduced size and increased power conversion efficiency is provided.