A switching power-supply device which includes a control unit configured to perform a switching control; and a resonance current detection unit configured to detect the resonance current flowing through the series resonance circuit.When an absolute value level of the resonance current exceeds a second threshold greater than a first threshold, the control unit inverts the turn on-and-off state of the first switching element or the second switching element at the time when the absolute value level reaches a third threshold between the first threshold and the second threshold. When the absolute value level of the resonance current exceeds the first threshold and does not exceed the second threshold, the control unit inverts the turn on-and-off state of the first switching element or the second switching element at the time when the absolute value level reaches the first threshold.