A backup system has a hard disk 16 connected to a terminal device 1 and accumulating data, a backup server 43 connected to the terminal device 1 through an IP network 3, a monitoring section 15e for monitoring on the terminal device 1 side a data occupancy ratio in the hard disk 16, an extracting section 15b for extracting data depending on an data attribute when the data occupancy ratio is greater than a predetermined level, an uploading section 15a for transmitting the extracted data to the backup server, a difference producing section 43b provided on the backup server 43 side and producing data of a difference between existing data having been accumulated and the uploaded data, a user database 44b for accumulating the produced difference data, and a downloading section 43e for producing data for download and transmitting the produced data to the terminal device 1, the data for download being produced from the existing data and the difference data. This structure can reduce a load on the backup server side when data accumulated in the terminal device are backed up by the backup server.