A planarized cover layer structure of a flexible circuit board includes an insulation layer bonded through a first adhesive layer to a surface of each one of conductive signal lines laid on a substrate of a flexible circuit board. Separation areas respectively formed between adjacent ones of the conductive signal lines are each formed with a filling layer, so that the filling layer provides the first adhesive layer with a planarization height in the separation areas and the planarization height is substantially equal to the height of the conductive signal lines. The filling layer can alternatively be of a height that is higher than the surface of the conductor layer by a covering height so that the first adhesive layer has a planarization height in the separation areas and the planarization height is substantially equal to the sum of the height of the conductive signal lines and the covering height.