A technique of backing up data for networked storage devices using de-duplication is disclosed in which a communication device divides a to-be-stored new file into data blocks, defines and updates a statistical value representative of a history of reference to each data block within previous files, and transmits the statistical value to another communication device. The communication device, upon reception of the statistical value, selects a preloaded data block, based on the received statistical value, and transmits to another communication device a copying request for making a copy of a real data block identical to the preloaded data block. The communication device, upon reception of the copy, stores the copy as the preloaded data block.