1. A memory module comprising: a system input/output terminal via which a system data signal having a predetermined data width is input/output; and a plurality of memory chips which transmit/receive an internal data signal broader than the system input/output terminal, the memory module further comprising: an IO chip including a function of performing conversion between the system data signal and the internal data signal in the system input/output terminal, the plurality of memory chips being stacked on the IO chip and being connected to the IO chip via through electrodes extending through the plurality of stacked memory chips.