A power supply device comprising: three resonant DC-DC converter phases (10-1, 10-2, 10-3), which are connected in parallel between an input node of a DC input voltage and an output node of a DC output voltage and of which operation phases are displaced from each other by 120 degrees, wherein the resonant DC-DC converter phases respectively include: switching circuits (11-1, 11-2, 11-3) which convert DC to AC by switching the DC input voltage by using first switching elements (Q11, Q12, Q21, Q22, Q31, Q32) respectively; serial resonant circuits (12-1, 12-2, 12-3) which are connected to output nodes of the switching circuits respectively; and rectifying-smoothing circuits (13-1, 13-2, 13-3) which are connected to output nodes of the serial resonant circuits respectively, the serial resonant circuits (12-1, 12-2, 12-3) respectively include first transformers (T11, T21, T31), second transformers (T12, T22, T32), and resonance capacitors, primary windings of the first transformers (T11, T21, T31), primary windings of the second transformers (T12, T22, T32), and the resonance capacitors are connected in series with each other respectively, secondary windings of the second transformers (T12, T22, T32) are connected to the rectifying-smoothing circuits respectively, the first transformers (T11, T21, T31) respectively include separate cores, and respectively have the primary and secondary windings isolated from each other with leakage inductances present between the primary and secondary windings, the secondary windings of different phases being connected in parallel with each other. three resonant DC-DC converter phases (10-1, 10-2, 10-3), which are connected in parallel between an input node of a DC input voltage and an output node of a DC output voltage and of which operation phases are displaced from each other by 120 degrees, wherein the resonant DC-DC converter phases respectively include: switching circuits (11-1, 11-2, 11-3) which convert DC to AC by switching the DC input voltage by using first switching elements (Q11, Q12, Q21, Q22, Q31, Q32) respectively; serial resonant circuits (12-1, 12-2, 12-3) which are connected to output nodes of the switching circuits respectively; and rectifying-smoothing circuits (13-1, 13-2, 13-3) which are connected to output nodes of the serial resonant circuits respectively, switching circuits (11-1, 11-2, 11-3) which convert DC to AC by switching the DC input voltage by using first switching elements (Q11, Q12, Q21, Q22, Q31, Q32) respectively; serial resonant circuits (12-1, 12-2, 12-3) which are connected to output nodes of the switching circuits respectively; and rectifying-smoothing circuits (13-1, 13-2, 13-3) which are connected to output nodes of the serial resonant circuits respectively, the serial resonant circuits (12-1, 12-2, 12-3) respectively include first transformers (T11, T21, T31), second transformers (T12, T22, T32), and resonance capacitors, primary windings of the first transformers (T11, T21, T31), primary windings of the second transformers (T12, T22, T32), and the resonance capacitors are connected in series with each other respectively, secondary windings of the second transformers (T12, T22, T32) are connected to the rectifying-smoothing circuits respectively, the first transformers (T11, T21, T31) respectively include separate cores, and respectively have the primary and secondary windings isolated from each other with leakage inductances present between the primary and secondary windings, the secondary windings of different phases being connected in parallel with each other.