Abstract: A transformer has an insulator interposed between respective primary and secondary coils wound on a coil bobbin which has an axial winding part and opposing radially extending flanges. The insulator is made from a continuous sheet of material which is corrugated to provide an insulating base having a width larger than the width of the winding part of the coil bobbin and from a continuous, plastic insulating sheet member having substantially the same width as the winding part. The corrugated insulated base material and insulating sheet member are wound between the primary coil and the secondary coil and the corrugated insulating base is bent at both side edges to be L-shaped to intimately fit to the inner surface of the flanges of the coil bobbin so as to provide an insulator which positively prevents coil wire at each end portion of the coil from dropping out of alignment with respective coil turns.