In the vicinity of the exposed part 1A of the multiply formed plural wiring conductors 1, of the thicknesses of the insulating cover layer 2 made of a polyimide layer, the thickness (T1) of a part located at about the middle point between the adjacent wiring conductors 1 is made smaller than the thickness (T2) of a part located on wiring conductor 1. Preferably, the difference in the thicknesses of a part located at about the middle point between the adjacent wiring conductors of the insulating cover layer 2 and that of a part located on wiring conductor is made to fall within the range of 1 5 .mu.m. As a result, the insulating cover layer does not peel off easily.