The inter-device fastening pillars DL1 to DL3 may be placed outside the top fastening pillars LU1 to LU4. There may be a plurality of inter-device fastening pillars DL1 to DL3. The plurality of inter-device fastening pillars may include, for example, a first inter-device fastening pillar DL1 placed outside the first top fastening pillar LU1 at the first corner CNR1, a second inter-device fastening pillar DL2 placed outside the second top fastening pillar LU2 at the second corner CNR2, and a third inter-device fastening pillar DL3 placed outside the third top fastening pillar LU3 at the third corner CRN3. In the case of the fourth corner CNR4 to which the memory connector MCN is placed adjacent, the inter-device fastening pillar may be omitted.