1. A semiconductor storage device comprising: a base substrate having a command/address external terminal group to which command signals and address signals are supplied, a data input/output external terminal group for inputting and outputting data signals, and a single chip select external terminal; and a plurality of memory chips that are stacked on the base substrate and are each individually capable of reading and writing operations, wherein a plurality of terminals constituting said command/address external terminal group, a plurality of terminals constituting said data input/output external terminal group, and said single chip select external terminal are connected to a single chip that has an interface function; and the single chip that has the interface function further has at least a chip select signal generation circuit that can individually activate said plurality of memory chips based on said address signals supplied via said command/address external terminal group and based on said chip select signal supplied via said chip select external terminal.