PURPOSE: To prevent a great increase in maintenance cost accompanying the generation of a program and the wasting of memory capacity and to improve the throughput of the controller by providing a general program which controls a fixed-length magnetic tape device.

CONSTITUTION: This controller is equipped with a variable-length read/write part 1 and a fixed-length read/write part 13 which respectively read and write data out of and on a variable-length and a fixed-length magnetic tape and channels 7 and 19 which read and write data out of and on each of the read/ write parts 1 and 13 in response to an input and an output request. Further, the controller is equipped with a file mark detecting means 17 which detects the file mark of the fixed-length magnetic tape where the data are read or written by the fixed length read/write part 13, and transmission control means 21 and 23 which store the file mark detected by this file mark detecting means 17, sends the number of remaining data on the fixed-length magnetic tape at the input/output request to the channel 19, and further transmits the stored file mark at the input/output request to the channel 19.