A backup system which can improve reliability of data without sacrificing availability of a data processing system and a duplicating apparatus employed in the backup system. The backup system includes a duplicating apparatus for taking in data in the transfer of the data and codes from a host apparatus to a storage apparatus and holding the data as duplicated data, the duplicating apparatus including a code fetching unit to fetch the codes and hold the fetched codes as duplicated codes, and a data verifying unit to verify existence or nonexistence of an error in the data by comparing the taken data and the duplicated data.