PURPOSE: To attain the free handling of various programs and to eliminate the waste of programs by performing the transfer of data in an optional unit by means of a storage which transfer the data in a fixed unit only.

CONSTITUTION: A data transfer device consists of a storage 11 which can trans fer the data in a fixed unit only, a storage 12 which can transfer the data in an optional unit, and a controller 15. When the 256-byte data are transferred to a storage 13 from the storage 11 having the 512-byte storage capacity, the 512 bytes, i.e., an entire sector of the hard disk 11 are transferred to the data transfer RAM 12. Then only the data to be transferred to the user memory 13 are transferred to a designated area among those data transferred to the RAM 12. Thus the 256-byte data can be transferred from the disk 11 without destructing the data of the memory 13 stored in the areas except the designated one. Then various programs can be freely handled.