A semiconductor device having a microcomputer chip and a plurality of high-speed memory chips and capable of making wiring lines of the memory chips equal in length is disclosed. The semiconductor device comprises a first wiring substrate, a microcomputer chip mounted over the first wiring substrate, a second wiring substrate disposed over the microcomputer chip, a plurality of first solder bumps for connecting the first and second wiring substrates with each other, and a plurality of second solder bumps as external terminals formed over a back surface of the wiring substrate. A first memory chip and a second memory chip, as high-speed memory chips, are stacked within the second wiring substrate, wiring of the first memory chip and that of the second memory chip are made equal in length within the second wiring substrate, and a completed package structure having the second wiring substrate is mounted over a completed package structure having the first wiring substrate.