A multilayer wiring board having a plurality of wiring layers is proposed, which is prepared by a process having the steps of: forming, on one insulating sheet, wiring patterns for all the wiring layers, which patterns are arranged at predetermined positions; and folding the insulating sheet having formed the wiring patterns in the predetermined order and stacking the folded sheet while positioning, and then heating the resultant sheet in a vacuum under a pressure to form a three-dimensional electric wiring.