Disclosed is a switching power supply device including: a transformer for voltage conversion; a main switching element connected in series to a primary coil; a primary side control circuit that performs on/off control of the main switching element; a synchronous rectification MOS transistor connected in series to a secondary coil; and a secondary side control circuit that performs on/off control of the MOS transistor, wherein the secondary side control circuit includes: an off-timing detection circuit that compares a drain voltage of the MOS transistor with a predetermined threshold voltage, and detects a timing to turn off the MOS transistor; and a threshold voltage setting circuit that sets the threshold voltage, and the threshold voltage setting circuit sets the threshold voltage based on a conduction period of the secondary coil or a period obtained by adding the conduction period to a conduction period of the primary coil.