A power supply device, comprising: a switching element in which switching operation is performed at high frequency; a transformer comprising primary and secondary windings (N1, N2), a power-supply voltage of a DC power supply being applied to the primary winding via the switching element; and a rectifying element connected in series to the secondary winding of the transformer, wherein a circuit element composed of at least one of the primary winding of the transformer and the switching element is connected to both ends of a series circuit, including the secondary winding of the transformer and the rectifying element, via first and second capacitors (C1, C2), wherein the both ends of the series circuit is connected to a load via an inductor, wherein the primary winding of the transformer, the first capacitor, the secondary winding of the transformer, and an input side terminal of the rectifying element are connected in series in that order, wherein the primary winding of the transformer and the secondary winding of the transformer are connected to the first capacitor with anti-polarity, wherein one end of the switching element is connected to a connecting point (a) of one end of the primary winding of the transformer and the first capacitor, wherein one end of the inductor is connected to a connecting point (b) of the first capacitor and the secondary winding of the transformer, wherein the second capacitor is connected between the other end of the switching element and an output side terminal of the rectifying element, wherein the other end of the primary winding of the transformer is connected to a positive electrode of the DC power supply, wherein a connecting point of the switching element and the second capacitor is connected to a negative electrode of the DC power supply, and wherein the load is connected between the other end of the inductor and a connecting point (c) of the rectifying element and the second capacitor. a switching element in which switching operation is performed at high frequency; a transformer comprising primary and secondary windings (N1, N2), a power-supply voltage of a DC power supply being applied to the primary winding via the switching element; and a rectifying element connected in series to the secondary winding of the transformer, wherein a circuit element composed of at least one of the primary winding of the transformer and the switching element is connected to both ends of a series circuit, including the secondary winding of the transformer and the rectifying element, via first and second capacitors (C1, C2), wherein the both ends of the series circuit is connected to a load via an inductor, wherein the primary winding of the transformer, the first capacitor, the secondary winding of the transformer, and an input side terminal of the rectifying element are connected in series in that order, wherein the primary winding of the transformer and the secondary winding of the transformer are connected to the first capacitor with anti-polarity, wherein one end of the switching element is connected to a connecting point (a) of one end of the primary winding of the transformer and the first capacitor, wherein one end of the inductor is connected to a connecting point (b) of the first capacitor and the secondary winding of the transformer, wherein the second capacitor is connected between the other end of the switching element and an output side terminal of the rectifying element, wherein the other end of the primary winding of the transformer is connected to a positive electrode of the DC power supply, wherein a connecting point of the switching element and the second capacitor is connected to a negative electrode of the DC power supply, and wherein the load is connected between the other end of the inductor and a connecting point (c) of the rectifying element and the second capacitor.