A first substrate having a first face and a second face is prepared. The first face has a plurality of product regions defined thereon. An electrode pad forming side of each of a semiconductor chip stack and a semiconductor chip is attached to each corresponding product region of the plurality of product regions. The second face of the first substrate is thinned. A first inorganic insulating layer is formed on the second face. A first vertical interconnection penetrates the first inorganic insulating layer and the first substrate and is electrically connected to an electrode pad of the semiconductor chip stack. A second vertical interconnection penetrates the first inorganic insulating layer and the first substrate and is electrically connected to an electrode pad of the semiconductor chip. A first horizontal interconnection electrically connects a part of the first vertical interconnection to a part of the second vertical interconnection.