PURPOSE: To prevent the disconstinuity of the wiring and thus eliminate defective boards, by bonding together in advance laminated bodies having an opening, sandwiched between an upper metal plate and a lower metal plate, by pressing, and then bonding the resultant laminated bodies together with a laminated body having no opening by pressing.

CONSTITUTION: Viaholes 2 are formed in a sheet 1, on which a wiring conductor layer 3 is in turn formed. Openings 4, 5 different in size are punched in the sheet 1. Sections containing an opening 4 of one size are laminated, and metal plates 11 having an opening of the same size are placed on the top and underside of the resultant laminated body 12; sections containing an opening 5 of the other size are laminated, and metal plates 11 having an opening of the same size are placed on the top and underside of the resultant laminated body 13. Similarly, sections of the sheet containing no opening are laminated, and metal plates 11' having no opening are placed on the top and underside of the resultant laminated body 14. Each of the laminated bodies 12, 13, 14 sandwiched between the metal plates 11, 11' is packaged in vacuum in a bag 8, and bonded by pressing using a medium 9. This eliminates steps. The laminated bodies 12, 13, 14 are taken out of the bags 8, and then laminated one another. The resultant laminated body 15 is packaged in vacuum in a bag 8, and bonded by pressing using the medium 9.