A switching power supply device comprising: a switching circuit configured to convert DC power supplied from a DC power supply into AC power; a rectifier circuit including a plurality of rectifier elements (Q5, Q6) configured to rectify the AC power resulting from conversion by the switching circuit; a snubber circuit including a power storage element configured to store therein surge power due to a reverse recovery time of the rectifier elements (Q5, Q6), a switching element configured to discharge the surge power stored in the power storage element, and an inductor provided in a stage following the switching element; and a controller configured to control an on-duty at which the switching element is turned on based on the surge power, characterized in that the snubber circuit has a continuous current mode in which an electric current flowing through the inductor continuously flows and a discontinuous current mode in which the electric current flowing through the inductor discontinuously flows; the controller is configured to set the on-duty of the switching element to a constant value (D max) in the continuous current mode, the controller is configured to control the switching element at a preset lower limit (Dth) indicating a lower limit of the on-duty, in the discontinuous current mode only, the on-duty lower limit (Dth) is smaller than the constant on-duty value (D max) set for the switching element in the continuous current mode and larger than 0, the on-duty lower limit (Dth) is equal to or larger than the on-duty of the switching element capable of outputting a snubber current minimum value (α) serving as a minimum value of an electric current output from the snubber circuit, and when the on-duty at which the switching element is turned on based on the surge power is smaller than the on-duty lower limit (Dth), the controller turns on the switching element at the on-duty lower limit (Dth). a switching circuit configured to convert DC power supplied from a DC power supply into AC power; a rectifier circuit including a plurality of rectifier elements (Q5, Q6) configured to rectify the AC power resulting from conversion by the switching circuit; a snubber circuit including a power storage element configured to store therein surge power due to a reverse recovery time of the rectifier elements (Q5, Q6), a switching element configured to discharge the surge power stored in the power storage element, and an inductor provided in a stage following the switching element; and a controller configured to control an on-duty at which the switching element is turned on based on the surge power, characterized in that the snubber circuit has a continuous current mode in which an electric current flowing through the inductor continuously flows and a discontinuous current mode in which the electric current flowing through the inductor discontinuously flows; the controller is configured to set the on-duty of the switching element to a constant value (D max) in the continuous current mode, the controller is configured to control the switching element at a preset lower limit (Dth) indicating a lower limit of the on-duty, in the discontinuous current mode only, the on-duty lower limit (Dth) is smaller than the constant on-duty value (D max) set for the switching element in the continuous current mode and larger than 0, the on-duty lower limit (Dth) is equal to or larger than the on-duty of the switching element capable of outputting a snubber current minimum value (α) serving as a minimum value of an electric current output from the snubber circuit, and when the on-duty at which the switching element is turned on based on the surge power is smaller than the on-duty lower limit (Dth), the controller turns on the switching element at the on-duty lower limit (Dth).