PURPOSE: To provide a data processing method which performs a readout operation without requiring excessive period of time other than a period of time required for readout even when the radout of the data stored in a library device is performed in an archival device.

CONSTITUTION: Two pieces of image data out of the image data comprised of several ten pieces of data in an archival device 6 are written on a magnetic disk 4, and residual data are stored in the library device 5. When a transmission request is issued, an optical disk is searched by the library device 5, and the data is sent out from the magnetic disk 4. When the delivery of the two pieces of data from the magnetic disk 4 is completed, following image data are sent out from the optical disk of the library device 5.