To easily adjust the balance of the current flowing to the side of the primary winding or secondary winding of a plurality of converter blocks.

The primary winding and the secondary winding are wound on a bobbin 33 thereby being fixed. Cores 31 and 33 can shift right and left. The position of a gap shifts to the primary winding side or the second winding side by shifting these cores 31 and 32. By the position of the gap shifting, the coupling capacitance between the primary winding and the secondary winding changes, and the leakage inductance of converter transformers 8a and 8b changes. Then, when the position where it comes to optimum leakage inductance is determined, the cores 31 and 32 are fixed.