Abstract: An internal composite layer for a multilayer circuit board, which is composed of an electrically insulating substrate with a copper layer formed thereon and a film of a coupling agent covering the copper layer is fabricated in accordance with the following method. That is, the copper layer is treated with a base or the base and an accelerating agent to supply hydroxyl groups to a surface thereof. Subsequently, thus treated copper layer is coated with the coupling agent, e.g., a silane coupling agent. The coupling agent is chemically bonded to the treated copper layer by the reaction of the hydroxyl groups with the coupling agent, so that an adhesion strength between the copper layer and the coupling agent is remarkably increased. In addition, since it is not necessary for forming a copper oxide layer in the surface of the copper layer, the internal composite layer shows excellent acid resistance without causing a pink-ring which usually occur around a through-hole plating.