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 based on the change of an equivalent circuit of 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; and a digital control circuit controlling the first switching element and the second switching element, characterized in that the digital control circuit sets on-times of the first switching element and the second switching element () with a timing based on a clock signal, on the basis of arithmetic processing, start timings of the on-times are determined with a timing based on the clock signal, at which the monitor signal is input as a trigger, and on the basis of this, a control signal used for turning on one of the first switching element and the second switching element is generated, and a stop timing of the on-time is determined with a timing based on the clock signal in accordance with the on-time set by the arithmetic processing, and on the basis of this, a control signal used for turning off the first switching element or the second switching element is generated, whereby a dead time is set on the basis of a load state and the digital control circuit generates a first switching control signal or a second switching control signal with maintaining a constant switching period (Ts). first monitor signal generation means for detecting the change of a voltage or current based on the change of an equivalent circuit of 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; and a digital control circuit controlling the first switching element and the second switching element, characterized in that the digital control circuit sets on-times of the first switching element and the second switching element () with a timing based on a clock signal, on the basis of arithmetic processing, start timings of the on-times are determined with a timing based on the clock signal, at which the monitor signal is input as a trigger, and on the basis of this, a control signal used for turning on one of the first switching element and the second switching element is generated, and a stop timing of the on-time is determined with a timing based on the clock signal in accordance with the on-time set by the arithmetic processing, and on the basis of this, a control signal used for turning off the first switching element or the second switching element is generated, whereby a dead time is set on the basis of a load state and the digital control circuit generates a first switching control signal or a second switching control signal with maintaining a constant switching period (Ts).