PURPOSE: To achieve a thinner converter circuit and improve production efficiency levels by forming print coils around insertion holes on a printed board and then hardening a liquid magnetic material that is inserted through the insertion holes and covers the print coils to form magnetic circuits.

CONSTITUTION: Appropriately shaped insertion holes 2, 3 are formed on a printed board 1. In the area surrounding the insertion holes 2, 3 on the printed board 1, the print coils 4, 5, which, for example, can serve as the primary and secondary windings for a transformer, are formed. On top of the print coil 5, a separate winding 6 is formed which serves as a tertiary winding with a few ten to a few thousand winding turns. Next, a magnetic material 7 is hardened to form a magnetic circuit that covers the print coils 4, 5 found on both sides of the printed board 1 and connects both sides of the board through the insertion holes 2, 3.