One or more through holes are formed by a process in a printed circuit board substrate formed of a resinous dielectric sheet and a conductive layer covering one surface of the dielectric sheet. The process involves the forming by laser one or more cavities on other surface of the dielectric sheet such that the cavities penetrate only the dielectric sheet, without penetrating the conductive layer. Both surfaces of the dielectric sheet are coated with a liquid photoresist layer such that the cavities are filled with the photoresist. A plurality of small areas are formed by photolithography on the surface which is covered with the conductive layer. The small areas are corresponding in location and shape to the cavities which may be of any shape. The small areas are stripped of the conductive, layer by etching before the cavities are stripped of the photoresist. The through holes are thus formed in the small areas defined by the cavities.