To provide a transformer and the assembling method of the same, improved in reliability by reducing the generation of abnormal sound of the transformer while contriving the improvement of the insulating property of a coil, resistance to shock, moisture-proofing and dust-proofing property of the transformer and preventing the generation of crack in a core piece even when the transformer is used in an environment wherein the change of temperature is violent.

The transformer is assembled by the assembling method consisting of a (process 2) for impregnating varnish into a site excluding a place which becomes a connecting part of first and second core pieces by dipping the site into varnish solution, a (process 7) for impregnating the varnish by dipping a bobbin consisting of a coiled coil into the varnish solution, a (process 11) for applying an adhesive agent on the bobbin, a (process 12) for connecting the first and second core pieces while pinching an insulating material by abutting the first core piece against the second core piece and a (process 14) for applying the adhesive agent on the connecting part of the first and second core pieces.