The reference numeral (2) designates a primary coil substrate formed by printing a coil pattern with resist on a commercial double-sided printed-circuit board and etching it, and the reference numeral (4) designates a secondary coil substrate formed in the same process. The reference numerals (1, 3 and 5) designate insulating boards. A flat transformer is produced by interposing adhesive sheets between these boards (1 to 5), compressing and heating them to form a laminated plate, punching the laminated plate into a desired shape, fitting a core (7) into the laminated plate punched, and fastening it by means of fasteners (8). The transformer can have desirable dielectric strength depending on the materials and thicknesses of the insulation plates (1, 3 and 5).