Two memory chips each being subjected to memory accesses in 2-bit units are assembled into a stacked structure by placing their back surfaces one over the other, so as to make memory accesses in 4-bit units. A memory module is so constructed that a plurality of such semiconductor storage devices, in each of which two memory chips each being subjected to memory accesses in 2-bit units are assembled into a stacked structure by placing their back surfaces one over the other, so as to make memory accesses in 4-bit units, are mounted on a mounting circuit board which is square and which is formed with electrodes along one latus thereof.