A DC-DC converter with an apparatus for controlling the DC-DC converter, the DC-DC converter comprising: a low-voltage port having a positive electrode terminal (16a1) and a negative electrode terminal (16a2); a high-voltage port having a positive electrode terminal (16b1) and a negative electrode terminal (16b2); a positive electrode wire connecting the positive electrode terminal (16a1) of the low-voltage port and the positive electrode terminal (16b1) of the high-voltage port; a negative electrode wire connecting the negative electrode terminal (16a2) of the low-voltage port and the negative electrode terminal (16b2) of the high-voltage port; a first common terminal (16k1) at which the positive electrode wire is branched; a second common terminal (16k2) at which the branched positive electrode wires are joined together; an inductor interposed between the positive, electrode terminal (16a1) of the low-voltage port and the first common terminal (16k1); a magnetic-field canceling type transformer including first and second windings (16e1, 16e2) wound in reverse directions from each other, each of the windings (16e1, 16e2) being interposed between the first common terminal (16k1) and the second common terminal (16k2) and connected to the first common terminal (16k1) at one end (16e1a, 16e2a); a first switching element (16f1) interposed between the other end (16e1b) of the first winding (16e1) and the negative electrode wire; a second switching element (16f2) interposed between the other end (16e2b) of the second winding (16e2) and the negative electrode wire; and diodes (16g3, 16g4) interposed between the other end (16e1b) of the first winding (16e1) and the second common terminal (16k2) and between the other end (16e2b) of the second winding (16e2) and the second common terminal (16k2), the apparatus comprising: current detecting means for detecting a current flowing through each of the first and second windings (16e1,16e2) at a portion (16h1) of the positive electrode wire connecting the second common terminal (16k2) with the positive electrode terminal (16b 1) of the high-voltage port; and control means for controlling operations of the first and second switching elements (16f1,16f2) based on the current detected by the current detecting means, characterized in that the transformer is configured such that a coupling degree, k, of the transformer is close to 1 to the maximum extent, the current detecting means comprises a single current sensor at the portion (16h1) of the positive electrode wire for detecting a first excitation current flowing through the second winding (16e2) due to mutual induction of the magnetic-field canceling type transformer when an exciting current flows through the first winding (16e1) under an ON state of the first switching element (16f1) and an OFF state of the second switching element (16f2) and a second excitation current flowing through the first winding (16e1) due to mutual induction of the magnetic-field canceling type transformer when an exciting current flows through the second winding (16e2) under an OFF state of the first switching element (16f1) and an ON state of the second switching element(16f2), and the control means calculates a current difference (ΔI) of the detected first excitation current and the detected second excitation current and controls the operations of the first and second switching elements (16f1, 16f2) such that the calculated current difference (ΔI) becomes 0. a low-voltage port having a positive electrode terminal (16a1) and a negative electrode terminal (16a2); a high-voltage port having a positive electrode terminal (16b1) and a negative electrode terminal (16b2); a positive electrode wire connecting the positive electrode terminal (16a1) of the low-voltage port and the positive electrode terminal (16b1) of the high-voltage port; a negative electrode wire connecting the negative electrode terminal (16a2) of the low-voltage port and the negative electrode terminal (16b2) of the high-voltage port; a first common terminal (16k1) at which the positive electrode wire is branched; a second common terminal (16k2) at which the branched positive electrode wires are joined together; an inductor interposed between the positive, electrode terminal (16a1) of the low-voltage port and the first common terminal (16k1); a magnetic-field canceling type transformer including first and second windings (16e1, 16e2) wound in reverse directions from each other, each of the windings (16e1, 16e2) being interposed between the first common terminal (16k1) and the second common terminal (16k2) and connected to the first common terminal (16k1) at one end (16e1a, 16e2a); a first switching element (16f1) interposed between the other end (16e1b) of the first winding (16e1) and the negative electrode wire; a second switching element (16f2) interposed between the other end (16e2b) of the second winding (16e2) and the negative electrode wire; and diodes (16g3, 16g4) interposed between the other end (16e1b) of the first winding (16e1) and the second common terminal (16k2) and between the other end (16e2b) of the second winding (16e2) and the second common terminal (16k2), current detecting means for detecting a current flowing through each of the first and second windings (16e1,16e2) at a portion (16h1) of the positive electrode wire connecting the second common terminal (16k2) with the positive electrode terminal (16b 1) of the high-voltage port; and control means for controlling operations of the first and second switching elements (16f1,16f2) based on the current detected by the current detecting means,