A DC power supply apparatus comprising: a DC voltage source; a primary circuit for converting the DC power of the DC voltage source into an AC power using the operations of switching elements (Q1 to Q4); a transformer for bringing in the AC power output by the primary circuit; a secondary circuit for converting the AC power output by the transformer into a DC power; a resonant circuit that is coupled to the DC output side of the secondary circuit in parallel, and that includes a series circuit of a resonant switching element (16, Qz) and a resonant capacitor; a filter circuit that includes a reactor and a capacitor, which is arranged to smooth DC power output by the secondary circuit; and a control device that is arranged to control the on/off-states of the switching elements (Q1 to Q4) of the primary circuit, characterized in that: when the voltage across the capacitor of the filter circuit is equal to or smaller than a predefined voltage value, the control device is arranged to keep one or more of the switching elements (Q1 to Q4)in the on-state so that a pulse voltage is being applied to the transformer whereby a current flowing through the reactor of the filter circuit becomes equal to or smaller than the maximum current value of the resonant current induced by the resonant element and the control device is further arranged to keep the resonant switching element in the on-state, and to keep one or more of the switching elements (Q1 to Q4) in the on-state so that a pulse voltage is being applied to the transformer and to keep the resonant switching element in the on-state a predefined number of times and to keep thereafter all of the switching elements (Q1 to Q4) and the resonant switching element in the off-state during a predefined time period, wherein the predefined time period is at least as long as the resonant cycle of the resonant switching element. a DC voltage source; a primary circuit for converting the DC power of the DC voltage source into an AC power using the operations of switching elements (Q1 to Q4); a transformer for bringing in the AC power output by the primary circuit; a secondary circuit for converting the AC power output by the transformer into a DC power; a resonant circuit that is coupled to the DC output side of the secondary circuit in parallel, and that includes a series circuit of a resonant switching element (16, Qz) and a resonant capacitor; a filter circuit that includes a reactor and a capacitor, which is arranged to smooth DC power output by the secondary circuit; and a control device that is arranged to control the on/off-states of the switching elements (Q1 to Q4) of the primary circuit, when the voltage across the capacitor of the filter circuit is equal to or smaller than a predefined voltage value, the control device is arranged to keep one or more of the switching elements (Q1 to Q4)in the on-state so that a pulse voltage is being applied to the transformer whereby a current flowing through the reactor of the filter circuit becomes equal to or smaller than the maximum current value of the resonant current induced by the resonant element and the control device is further arranged to keep the resonant switching element in the on-state, and to keep one or more of the switching elements (Q1 to Q4) in the on-state so that a pulse voltage is being applied to the transformer and to keep the resonant switching element in the on-state a predefined number of times and to keep thereafter all of the switching elements (Q1 to Q4) and the resonant switching element in the off-state during a predefined time period, wherein the predefined time period is at least as long as the resonant cycle of the resonant switching element.