A DC-DC converter, comprising: a full bridge circuit, composed of a first switching leg connecting a first and a second switching element (S1, S2) in series, and a second switching leg connecting a third and a fourth switching element (S3, S4) in series, the first and second switching legs being connected in parallel, wherein DC terminals are provided between both ends of the first switching leg and between both ends of the second switching leg, and AC terminals are provided between the series connecting point of the first and the second switching elements, and the series connecting point of the third and the fourth switching elements; a rectifier circuit having a smoothing reactor; a smoothing capacitor connected to a load in parallel, and connected to DC terminals of the rectifier circuit; a primary winding connected to the AC terminals of the full bridge circuit; a secondary winding connected to AC terminals of the rectifier circuit; a transformer for connecting magnetically the primary winding and the secondary winding; and a control means for controlling the full bridge circuit, wherein the first, second, third and fourth switching elements are each composed of a switch, an antiparallel diode (DS1, DS2, DS3, DS4) connected to the switch in parallel, and a capacitor (CS1, CS2, CS3, CS4) connected to the switch and the antiparallel diode in parallel, the DC-DC converter having a reactor component inserted between the AC terminals and the primary winding of the full bridge circuit in series; when the power supply amount to the load is a predetermined value or more, the control means implements a first mode for making the first, the second, the third and the fourth switching elements driven; and when the power supply amount to the load is a predetermined value or less, the control means implements a second mode for making the switching elements of one of the first switching leg and the second switching leg stopped, in an OFF state, and for making the switching elements of the other of the first switching leg and the second switching leg driven, characterised by another smoothing capacitor, connected to a DC power source in parallel and connected to the DC terminals of the full bridge circuit, wherein the antiparallel diodes connected to the stopped switching elements of the one switching leg when implementing the second mode, have a reverse recovery characteristic slower than that of the antiparallel diodes connected to the driven switching elements of the other switching leg when implementing the second mode. a full bridge circuit, composed of a first switching leg connecting a first and a second switching element (S1, S2) in series, and a second switching leg connecting a third and a fourth switching element (S3, S4) in series, the first and second switching legs being connected in parallel, wherein DC terminals are provided between both ends of the first switching leg and between both ends of the second switching leg, and AC terminals are provided between the series connecting point of the first and the second switching elements, and the series connecting point of the third and the fourth switching elements; a rectifier circuit having a smoothing reactor; a smoothing capacitor connected to a load in parallel, and connected to DC terminals of the rectifier circuit; a primary winding connected to the AC terminals of the full bridge circuit; a secondary winding connected to AC terminals of the rectifier circuit; a transformer for connecting magnetically the primary winding and the secondary winding; and a control means for controlling the full bridge circuit, wherein the first, second, third and fourth switching elements are each composed of a switch, an antiparallel diode (DS1, DS2, DS3, DS4) connected to the switch in parallel, and a capacitor (CS1, CS2, CS3, CS4) connected to the switch and the antiparallel diode in parallel, the DC-DC converter having a reactor component inserted between the AC terminals and the primary winding of the full bridge circuit in series; when the power supply amount to the load is a predetermined value or more, the control means implements a first mode for making the first, the second, the third and the fourth switching elements driven; and when the power supply amount to the load is a predetermined value or less, the control means implements a second mode for making the switching elements of one of the first switching leg and the second switching leg stopped, in an OFF state, and for making the switching elements of the other of the first switching leg and the second switching leg driven, characterised by another smoothing capacitor, connected to a DC power source in parallel and connected to the DC terminals of the full bridge circuit, wherein the antiparallel diodes connected to the stopped switching elements of the one switching leg when implementing the second mode, have a reverse recovery characteristic slower than that of the antiparallel diodes connected to the driven switching elements of the other switching leg when implementing the second mode.