A host device (1) adds host function information indicating a function of the host device (1) to a command and transmits it to a storage device (2). The storage device (2) receives the command transmitted from the host device (1) and determines whether the function indicated by the host function information can be processed. When the function can be processed, the function is processed based on the command. When the function cannot be processed, no response is returned or error information is returned. Thus, under simple control, it is possible to prevent data destruction by a host device of an old version.