PURPOSE: To make it easy to make a primary winding and improve its space factor by making the primary winding of a sheet-shaped wire consisting of thin insulated wires arranged and bonded into sheet form and turned.

CONSTITUTION: A primary winding 15 comprises a sheet-shaped wire 19 consisting of many thin insulated wires, such as enamel wires, arranged and bonded into sheet form and wound around a bobbin 13 in many folds. Both ends of the primary winding 15 are drawn downward through a base 16, passed through slits 18, and soldered. In soldering, the enamel at both ends of the primary winding 15 (sheet-shaped wire 19) is peeled to expose the conductors of the enamel wires composing the sheet-shaped wire 19 and both ends thereof are passed through the slits 18 and soldered to lands. For making the primary winding 15, only winding of the sheet-shaped wire 19 around the bobbin 13 in many folds is required. This work is easier than winding of a cable-shaped litz wire 5 into coil form.