There is provided a low-cost semiconductor device that commercial and quality-assured (inspected) chip size packages can be stacked and has a small co-planarity value and a high mounting reliability. A semiconductor device in which a flexible circuit substrate is adhered to at least a part of a lateral side of a semiconductor package, and the flexible circuit substrate, which is on a side facing solder balls of the semiconductor package, is folded at a region 25 inside of an edge of the semiconductor package and also at a region 26 outside of the outermost solder balls 24 on the semiconductor package