PURPOSE: To attain rewriting without giving any effect on blocks before and after the erased block, by driving a tape reversingly for one block's share, measuring a tape length l1 of the block, writing the write data from the head of said block, measuring the tape length l2 of the block written newly, and erasing the contents on the tape by the tape length of l1-l2.

CONSTITUTION: When a circuit 1 receives a BSW command, the operation of the back space of a magnetic tape is instructed to an MTU control circuit 7. The back space is done until an IBG is detected, the number of tachometer pulses until the IBG is detected with a circuit 3 is counted and stored in a register 5. Position information is transmitted from the circuit 4 to the circuit 7 through the detection of the IBG to stop the running of the magnetic tape. Then, the magnetic tape is advanced forward to obtain a specified IBG length, and a write head is set on with the circuit 7, and the write data is written by a designated length. In this case, the number of tachogenerator pulses stored in the register 5 is compaired with the number of tachometer pulses counted at the write. When the data length instructed with a READ command and a BSW command are coincident, the position information is given to the circuit 7 when the tachogenerator pulse numbers are coincident, to set off a WRITE head, allowing to complete the processing of the BSW command.