A bidirectional DC-DC converter comprising: a first switching circuit (H2, H2, H3, H4) to input electrical power from a first DC power supply, with which a first smoothing capacitor is connected in parallel, to an in-between terminal of DC terminals of the first switching circuit and to convert the electrical power to AC so as to supply the AC from an in-between terminal of AC terminals to a primary winding; a second switching circuit (S1, S2, S3, S4) to input electrical power from a second DC power supply, with which a second smoothing capacitor is connected in parallel, to an in-between terminal of DC terminals of the second switching circuit and to convert the electrical power to AC so as to supply the AC from an in-between terminal of AC terminals to a secondary winding; a transformer which magnetically couples the primary winding to the secondary winding; and control means for controlling the first and second switching circuits so as to transfer electrical power between the first and second DC power supplies (V1, V2), wherein the first switching circuit includes: a first switching leg in which switching devices (H1 and H2) are connected in series; and a second switching leg in which switching devices (H3 and H4) are connected in series and which is connected in parallel with the first switching leg, wherein an in-between point of both ends of the first switching leg is the in-between terminal of the DC terminals while an interval between a point at which the switching devices (H1 and H2) are connected in series and a point at which the switching devices (H3 and H4) are connected in series is the in-between terminal of the AC terminals, and wherein the control means is in a mode A configured to keep an ON state of the switching devices (H2 and H3) upon supplying electrical power from the second DC power supply to the first DC power supply, characterized in that the control means is in a mode B configured to switch a state of the switching devices (S1,S2,S3,S4) provided with the second switching circuit over to supply electrical power of the second DC power supply to the secondary winding with the ON state of the switching devices (H2 and H3) being intact. a first switching circuit (H2, H2, H3, H4) to input electrical power from a first DC power supply, with which a first smoothing capacitor is connected in parallel, to an in-between terminal of DC terminals of the first switching circuit and to convert the electrical power to AC so as to supply the AC from an in-between terminal of AC terminals to a primary winding; a second switching circuit (S1, S2, S3, S4) to input electrical power from a second DC power supply, with which a second smoothing capacitor is connected in parallel, to an in-between terminal of DC terminals of the second switching circuit and to convert the electrical power to AC so as to supply the AC from an in-between terminal of AC terminals to a secondary winding; a transformer which magnetically couples the primary winding to the secondary winding; and control means for controlling the first and second switching circuits so as to transfer electrical power between the first and second DC power supplies (V1, V2), wherein the first switching circuit includes: a first switching leg in which switching devices (H1 and H2) are connected in series; and a second switching leg in which switching devices (H3 and H4) are connected in series and which is connected in parallel with the first switching leg, a first switching leg in which switching devices (H1 and H2) are connected in series; and a second switching leg in which switching devices (H3 and H4) are connected in series and which is connected in parallel with the first switching leg, wherein an in-between point of both ends of the first switching leg is the in-between terminal of the DC terminals while an interval between a point at which the switching devices (H1 and H2) are connected in series and a point at which the switching devices (H3 and H4) are connected in series is the in-between terminal of the AC terminals, and wherein the control means is in a mode A configured to keep an ON state of the switching devices (H2 and H3) upon supplying electrical power from the second DC power supply to the first DC power supply, characterized in that the control means is in a mode B configured to switch a state of the switching devices (S1,S2,S3,S4) provided with the second switching circuit over to supply electrical power of the second DC power supply to the secondary winding with the ON state of the switching devices (H2 and H3) being intact.