PURPOSE: To eliminate the time loss of position allocation and to add and record a data block efficiently by recording specific information and a specific identification number additionally when the data block is recorded in a data file additionally.

CONSTITUTION: When the data block is written in the data file additionally, a magnetic head is moved and stopped at a position (d) where the 1st tape mark is detected. Further, the magnetic head is positioned at a point (c), and when it is recognized that the data read in at this time is a tape mark 2, the U flag in the control flag as the 4th byte of a residual data group 22 is set to 1 and the W flag is set to 0. A write command is generated at this point of time to read the last data block 3, and when it is judged that it is at distance much longer than specific length, writing operation starts immediately. Then, the U flag is set to 1 to write the 1st data block 3, and the next data block 3 is written by setting the U flag to 0.