In a method of manufacturing a semiconductor device of the invention, a rigid substrate which supports one or more semiconductor elements on a surface of the substrate and is clamped between an upper mold and a lower mold of an encapsulation mold at a time of resin encapsulation is provided, so that a vent-end edge portion of the substrate corresponding to a vent end of the encapsulation mold has a thickness smaller than a thickness of other portions of the substrate. The substrate is disposed in the encapsulation mold, and resin is injected into a cavity between the upper mold and the substrate to encapsulate the semiconductor elements with the resin.