A multi-chip semiconductor device includes a plate-shaped first semiconductor chip having a first connection portion in which a first semiconductor chip electrode is formed on a first main surface of the first semiconductor chip or on a first side surface vertical to the first main surface, and a plate-shaped second semiconductor chip having a second connection portion in which a second semiconductor chip electrode is formed on a second side surface vertical to a second main surface of the second semiconductor chip. Each of the first and second connection portions includes at least an inclined surface that is inclined with respect to each of the first and second main surfaces. The first connection portion and the second connection portion are connected to each other such that the first main surface of the first semiconductor chip and the second main surface of the second semiconductor chip are vertical to each other.