An object is to make it possible to add a vendor-unique command at the time at which addition of a vendor-unique command becomes necessary later even in the case where the device side does not have a dedicate pin for updating in communication control based on a SATA standard. An information processing system that performs data communication between a host and a device in conformity with the SATA standard, and the host transmits a setup command to which information on an undefined command is written to the device, and the device: has a command table for commands in conformity with the SATA standard, in which a command code to identify each command and information on a transfer protocol of each command are described; and makes the undefined command available between the host and the device by writing information on the undefined command to the command table in accordance with the received setup command.