A novel switching power supply device no more in need of a stand-by signal for turning on/off a switching element is provided. The switching power supply device is configured as having a main switching element Q1, wherein the switching power supply circuit is connected with a pulse oscillation circuit IC1 for outputting pulse signal; the pulse oscillation circuit and an output terminal of the main switching element are connected with an input section of an internal detection circuit 12; the internal detection circuit 12 is configured so as to control timing of switching of the main switching element Q1 upon detection of the pulse signal supplied from the pulse oscillation circuit IC1.