A pair of flanges 16 are formed on both ends of a spool 14 of a bobbin 12, and an insulation layer 32 formed by heat curing an interlayer insulation prepreg tape 22 is formed on the external periphery of the spool 14. A primary coil 26 is wound on the external periphery of the insulation layer 32, and a secondary coil 28 is wound on the external periphery of the primary coil 26. Insulation layers 32 are provided respectively between the primary coil 26 and the secondary coil 28, and on the external periphery of the secondary coil 28. Insulation layers 34 formed by heat curing an inner wall insulation prepreg tape 24 are provided on mutually opposed faces 16A of the paired flanged 16, and such insulations layers 32 and 34 have a mutually linked structure.
