To make it unnecessary to arrange a land for soldering on the outside of a case, by fitting a metal fixture which is previously fixed on a printed board into a case holding a transformer element.

A transformer element (a) is first built in a case (b), and engaged with four pawl-shaped protrusions b-1 of the case (b) which are arranged at portions corresponding to node points a-1. At this time, on the upper surface and the insides of the side surfaces in the widthwise direction of the case (b), belt-shaped ribs are arranged at portions corresponding to the node points a-1. The transformer element (a) is brought into contact with the case (b), and these are held only by using the ribs and the protrusions b-1. On a printed board (c), two metal fixtures (d) for fixing are fixed on lands c-1 by soldering. The case (b) in which the transformer element (a) is built is attached on the printed board (c). At this time, protrusions b-2 of the case (b) are engaged with square holes positioned at upright parts of the metal fixtures (d) of the printed board (c), and the transformer element (a) is fixed on the printed board (c).