A COC DRAM including a plurality of stacked DRAM chips is mounted on a motherboard by using an interposer. The interposer includes a Si unit and a PCB. The Si unit includes a Si substrate and an insulating-layer unit in which wiring is installed. The PCB includes a reference plane for the wiring in the Si unit. The wiring topology between a chip set and the COC DRAM is the same for every signal. Accordingly, a memory system enabling a high-speed operation, low power consumption, and large capacity is provided.