A sheet coil in the form of a conductor on an insulating substrate has a mending metal portion at a side portion forming the conductor gap so as to narrow the gap width. The sheet coil can be produced by forming a coil pattern on a base material made up of an insulating substrate and a metal layer laminated thereon by using a photosensitive material. The portion of the metal layer not covered with the photosensitive material is removed by etching, and, while the photosensitive material remains, a metal plating treatment is carried out so as to narrow the etched width of the metal layer.