An input/output (IO) system reduces the processing load involved in judging whether a device is valid or revoked. The system is constituted from an input/output (IO) device and an information usage device. The IO device outputs an identifier (ID) list to the information usage device, the ID list including one or more identifiers (IDs), arranged according to a predetermined rule, that each correspond to a different valid or revoked device. The information usage device uses the received ID list to specify a target range that includes a target ID stored by the information usage device, and outputs range information indicating the specified target range to the IO device, which uses the received range information in judging whether the information usage device is valid or revoked.