PURPOSE: To shorten the delivery term remarkably by a method wherein, when the ion implantation type ROM is formed, they are protected by insulating film and stored after forming each region of source, drain and wiring layer, and the conductive layers between regions are formed by ion implantation whenever an order is placed.

CONSTITUTION: The surface of P type Si substrate 1 is coated with SiO2 film 24 and the Si3N4 film 25 corresponding to the source, drain and wiring layer region is formed on the film 24 while the thick field oxide film 12 is formed between the films 24 and 25 by means of heat treatment. Next the film 25 is removed and the multicrystal Si layer 173 and PSG film 26 are laminated on the overall surface including the gate film 16 comprising the remaining film 24, while the source drain and wiring regions not shown in the drawing are formed by means of heat treatment to diffuse the P impurities contained in the film 26. Then the film 26 is removed and the exposed surface is coated with SiO2 film 19 and Al wiring 22 is provided to be stored as it is. When an order is placed, the resist film 27 with opening 20 is formed on the film 19 and the N+ type region 18 making source and drain conductive is formed by means of the ion implantation.