A switching power supply device which converts an input power input from an AC power source to supply the power to a load, the switching power supply device comprising: a power factor improvement circuit; a phase-shifted full bridge type DC/DC converter that is arranged in a rear stage of the power factor improvement circuit and has a full-bridge type switching circuit; an output current detecting circuit for detecting an output current to be supplied to the load; an output voltage detecting circuit for detecting an output voltage to be supplied to the load; a power factor improvement circuit output voltage detecting circuit for detecting a power factor improvement circuit output voltage, which is input to the DC/DC converter from the power factor improvement circuit; and a control unit for dynamically changing dead times of the full-bridge type switching circuit based on the power factor improvement circuit output voltage, the output current, and the output voltage, wherein the control unit applies the dead times that have been changed to control switching of the full-bridge type switching circuit, wherein the power factor improvement circuit is an active type power factor improvement circuit having a switching element (S11, S12) of which switching is controlled by the control unit, wherein the switching power supply device comprises: an input current detecting circuit for detecting an input current input from the AC power source; and an input voltage detecting circuit for detecting an input voltage input from the AC power source, and wherein the control unit controls the switching element (S11, S12) of the power factor improvement circuit such that the power factor improvement circuit output voltage becomes a target voltage determined based on the input current, the input voltage, the output current, and the output voltage, and dynamically changes the dead times based on the power factor improvement circuit output voltage, the output current, and the output voltage. a power factor improvement circuit; a phase-shifted full bridge type DC/DC converter that is arranged in a rear stage of the power factor improvement circuit and has a full-bridge type switching circuit; an output current detecting circuit for detecting an output current to be supplied to the load; an output voltage detecting circuit for detecting an output voltage to be supplied to the load; a power factor improvement circuit output voltage detecting circuit for detecting a power factor improvement circuit output voltage, which is input to the DC/DC converter from the power factor improvement circuit; and a control unit for dynamically changing dead times of the full-bridge type switching circuit based on the power factor improvement circuit output voltage, the output current, and the output voltage, wherein the control unit applies the dead times that have been changed to control switching of the full-bridge type switching circuit, wherein the power factor improvement circuit is an active type power factor improvement circuit having a switching element (S11, S12) of which switching is controlled by the control unit, wherein the switching power supply device comprises: an input current detecting circuit for detecting an input current input from the AC power source; and an input voltage detecting circuit for detecting an input voltage input from the AC power source, and an input current detecting circuit for detecting an input current input from the AC power source; and an input voltage detecting circuit for detecting an input voltage input from the AC power source, and wherein the control unit controls the switching element (S11, S12) of the power factor improvement circuit such that the power factor improvement circuit output voltage becomes a target voltage determined based on the input current, the input voltage, the output current, and the output voltage, and dynamically changes the dead times based on the power factor improvement circuit output voltage, the output current, and the output voltage.