A current resonant type DC voltage converter, comprising: a switching unit configured to generate an AC voltage from a DC input voltage (V in); a resonance unit configured to resonate in response to application of the AC voltage to a resonant capacitance and a first resonant inductance; a transformer having a primary side thereof connected in series to the resonance unit; a second resonant inductance present in parallel to the transformer; a rectifier configured to rectify a current appearing on a secondary side of the transformer to generate a DC output voltage; a parallel resonant inductance adjustment unit configured to vary the second resonant inductance, characterized in that the current resonant type DC voltage converter further comprises a voltage detection unit configured to detect the input voltage (V in) or the output voltage (V out), wherein the parallel resonant inductance adjustment unit has an adjusting inductance connected in parallel on the primary side of the transformer via a second switch, the current resonant type DC voltage converter further comprising: a second switch controller configured to control switching of the second switch based on the input voltage (V in) or the output voltage (V out) detected by the voltage detection unit, wherein the second switch controller has a transition condition voltage range fixed as a condition for a switching state transition of the second switch, the second switch controller conducts control to open the second switch when the input voltage (V in) or the output voltage (V out) is within the transition condition voltage range and close the second switch when the input voltage (V in) or the output voltage (V out) is not within the transition condition voltage range, and the second switch controller varies the transition condition voltage range for the second switch being closed with respect to the transition condition voltage range for the second switch being opened to make switching control of the second switch have a hysteresis. a switching unit configured to generate an AC voltage from a DC input voltage (V in); a resonance unit configured to resonate in response to application of the AC voltage to a resonant capacitance and a first resonant inductance; a transformer having a primary side thereof connected in series to the resonance unit; a second resonant inductance present in parallel to the transformer; a rectifier configured to rectify a current appearing on a secondary side of the transformer to generate a DC output voltage; a parallel resonant inductance adjustment unit configured to vary the second resonant inductance, characterized in that the current resonant type DC voltage converter further comprises a voltage detection unit configured to detect the input voltage (V in) or the output voltage (V out), wherein the parallel resonant inductance adjustment unit has an adjusting inductance connected in parallel on the primary side of the transformer via a second switch, the current resonant type DC voltage converter further comprising: a second switch controller configured to control switching of the second switch based on the input voltage (V in) or the output voltage (V out) detected by the voltage detection unit, wherein the second switch controller has a transition condition voltage range fixed as a condition for a switching state transition of the second switch, the second switch controller conducts control to open the second switch when the input voltage (V in) or the output voltage (V out) is within the transition condition voltage range and close the second switch when the input voltage (V in) or the output voltage (V out) is not within the transition condition voltage range, and the second switch controller varies the transition condition voltage range for the second switch being closed with respect to the transition condition voltage range for the second switch being opened to make switching control of the second switch have a hysteresis. a second switch controller configured to control switching of the second switch based on the input voltage (V in) or the output voltage (V out) detected by the voltage detection unit, wherein the second switch controller has a transition condition voltage range fixed as a condition for a switching state transition of the second switch, the second switch controller conducts control to open the second switch when the input voltage (V in) or the output voltage (V out) is within the transition condition voltage range and close the second switch when the input voltage (V in) or the output voltage (V out) is not within the transition condition voltage range, and the second switch controller varies the transition condition voltage range for the second switch being closed with respect to the transition condition voltage range for the second switch being opened to make switching control of the second switch have a hysteresis.