The reliability of a semiconductor device is enhanced. A first lead frame, a first semiconductor chip, a second lead frame, and a second semiconductor chip are stacked over an assembly jig in this order with solder in between and solder reflow processing is carried out to fabricate their assembly. Thereafter, this assembly is sandwiched between first and second molding dies to form an encapsulation resin portion. The upper surface of the second die is provided with steps. At a molding step, the second lead frame is clamped between the first and second dies at a position higher than the first lead frame; and a third lead frame is clamped between the first and second dies at a higher position. The assembly jig is provided with steps at the same positions as those of the steps in the upper surface of the second die in positions corresponding to those of the same.