A flat cable includes first and second insulator sheets, and a plurality of conductor elements arranged in parallel relation to one another over the length of the sheets. The first insulator sheet is provided with an adhesive layer. The conductor elements are interposed between the adhesive layer and the second insulator sheet. The first and second insulator sheets are first press-adhered under heat through the adhesive layer, and then bonded by an ultrasonic welding unit. The ultrasonic welding unit includes a horn for imparting ultrasonic oscillations, and an anvil placed in opposition to the horn. The first and second insulator sheets are bonded in the zones which extend along the length of the sheets and are located outside the loci where the conductor elements are arranged. The bonding is performed either continuously or in an intermittent manner. In a flat cable thus produced, the conductor elements embedded therein are efficiently prevented from mutual short-circuiting. Although the adhesive layer is made very thin, the insulator sheets are adhered to each other very firmly.