A pseudo-resonant switching power source device is provided which comprises a primary winding 2a of a transformer 2 and a main MOS-FET 3 connected in series to a DC power source 1, a voltage-resonant capacitor 12 connected in parallel to main MOS-FET 3, a rectification smoother 4 connected to a secondary winding 2b of transformer 2 and having a rectification MOS-FET 51 and a smoothing capacitor 6, a synchronized rectification controller 52 for turning rectification MOS-FET 51 on after turning-off of main MOS-FET 3 and turning rectification MOS-FET 51 off before turning-on of main MOS-FET 3, a pulse width elongation circuit 55 for extending on-pulse width of rectification MOS-FET 51, and a main control circuit 8 for producing main drive signals to turn main MOS-FET 3 off and on with operation frequency responsive to on-pulse width of rectification MOS-FET 51 extended by pulse width elongation circuit 55 to vary operation frequency of main MOS-FET 3 so as to restrain noise by high frequency components in main drive signals from main control circuit 8.