A switching power supply apparatus including a direct-current power-supply input unit to which a direct-current input voltage (Vi) is input, a transformer being configured by one magnetic component and including at least a first primary winding (np) and a first secondary winding, magnetically coupled, an inductor (Lr) connected in series to the first primary winding (np), a first switch circuit including a parallel circuit of a first switching element, a first capacitor, and a first diode, a second switch circuit including a parallel circuit of a second switching element, a second capacitor, and a second diode, a third capacitor (Cr), a first series circuit that is connected to both end portions of the direct-current power-supply input unit and in which the first primary winding (np) and the first switch circuit are connected in series, and a second series circuit that is connected to both end portions of the first switch circuit or both end portions of the first primary winding (np) and in which the second switch circuit and the third capacitor (Cr) are connected in series, wherein the first switch circuit and the second switch circuit are configured so as to operate so that the first switch circuit and the second switch circuit complementarily repeat turnon and turnoff with a time period in between, for which both the first switch circuit and the second switch circuit are turned off, and the switching power supply apparatus includes a power converter circuit configured so that an output voltage (Vout) is output to a secondary side through a first rectification smoothing circuit rectifying and smoothing an alternating-current voltage output from the first secondary winding, the switching power supply apparatus comprising: first monitor signal generation means for detecting the change of a voltage or current in the power converter circuit, which occurs owing to the turnoff of a switch circuit in an on-state from among the first switch circuit and the second switch circuit, and generating a monitor signal; output voltage detection means for detecting the output voltage (Vout); feedback signal generating means for generating a feedback signal according to the output voltage (Vout) detected by the output voltage detection means; and a switching control circuit controlling the first switching element and the second switching element, characterized in that the switching control circuit includes an oscillator generating a triangular wave whose frequency is settable and nearly constant, a first comparator comparing the triangular wave signal with the feedback signal, and a second comparator comparing an output signal of the first monitor signal generation means with a first threshold value, wherein a gate signal of the first switch circuit and a gate signal of the second switch circuit are generated on the basis of outputs of the first comparator and the second comparator, wherein the feedback signal is used to obtain the turn off timing according to the level of the output voltag (Vout) and the switching control circuit is arranged to generate a first switching control signal and a second switching control signal with maintaining a constant switching period (Ts) so that a desired output voltage level is obtained. first monitor signal generation means for detecting the change of a voltage or current in the power converter circuit, which occurs owing to the turnoff of a switch circuit in an on-state from among the first switch circuit and the second switch circuit, and generating a monitor signal; output voltage detection means for detecting the output voltage (Vout); feedback signal generating means for generating a feedback signal according to the output voltage (Vout) detected by the output voltage detection means; and a switching control circuit controlling the first switching element and the second switching element, characterized in that the switching control circuit includes an oscillator generating a triangular wave whose frequency is settable and nearly constant, a first comparator comparing the triangular wave signal with the feedback signal, and a second comparator comparing an output signal of the first monitor signal generation means with a first threshold value, wherein a gate signal of the first switch circuit and a gate signal of the second switch circuit are generated on the basis of outputs of the first comparator and the second comparator, wherein the feedback signal is used to obtain the turn off timing according to the level of the output voltag (Vout) and the switching control circuit is arranged to generate a first switching control signal and a second switching control signal with maintaining a constant switching period (Ts) so that a desired output voltage level is obtained.