Object of this invention is to prevent deviating increase of switching frequency of a switching element for a half-wave rectification current resonance type switching power supply device in the process of a soft start operation. To solve the object, between output terminals of a direct current power source, a first switching element Q1 and a second switching element Q2 are connected in series. A primary winding of a transformer T1 and a current resonant capacitor Cri are connected to the second switching element Q2 in parallel. A gate control circuit is designed to gradually lengthen the on period of the first switching element Q1 and also designed to shorten the on period of the second switching element Q2 in time of the start operation. This makes it possible to perform a soft start without deviating from the frequency band required for voltage control.