PURPOSE: To prevent a ferrite cord from being subjected to thermal effect when a preliminary soldering is conducted by a method wherein a mounting hole, where a ferrite core to be attached to a coil will be passed through, is provided on a circuit board.

CONSTITUTION: An oblong hole 21 is formed in the transformer-mounting part 16 of a circuit board 11, and a plurality of through holes 14, where the coil-terminal pins 20 of a transformer 19 will be inserted, connected and fixed, are provided in the vicinity of both sides of the above-mentioned hole 21. The transformer 19 is mounted in such a manner that the terminal pin 20 of a coil 17 is inserted into the above-mentioned through hole 14, the coil 17 is placed on one surface of the circuit board 11 and the transformer 19 is supported. Other electric parts are supported by the board 11 in the same manner as above, and at the same time, a preliminary soldering is conducted using a solder-dipping method. The side 18a of ferrite cores 18a and 18b is inserted into the square hole 21 of the circuit board 11, the center leg 23 is inserted into the inserting hole 22 of the primary coil, the coil 17 is fitted into a coil-fitting part 24, and both side legs are inserted. Other ferrite core 18b is installed from the upper side of the coil, and the center leg 23 and both side legs are connected by filling in the space between the legs with epoxy resin material.