To obtain a transformer having a good insulating property by sufficiently filling resin into gaps between inner and outer coils.

In the transformer, inner and outer coils 3 and 2 formed by providing windings 12 and 13 around outer peripheries of cylindrical barrel parts 21 and 31 of bobbins 20 and 30 are arranged concentrically, a core 1 is inserted in an inner diameter side of the inner coil 3, and insulating resin melt is filled around the windings 12 and 13 of the inner and outer coils 2 and 3. Resin filling ports 4, 5 and 6 communicating with an annular space S1 defined between the inner and outer bobbin barrel parts 21 and 31 are provided in at least one of bobbin end walls 24, 31 and 32 blocking the space S1 from its shaft end side.