To surely update a firmware in a short time by extending a data compressed firmware (control program) which is supplied from an external device through a data extending circuit and writing it in a nonvolatile memory thereafter.

An external device compresses data with the same algorithm as the data for compressing circuit 1 of a magnetic tape unit A and transfers a firmware, to which an error check code is added, via a serial interface(I/F) part 8 to a data buffer 3. A CPU 7 extends the firmware via a data expanded circuit 2 and stores the expanded firmware in a RAM 9. When no error is detected and if the error check code is normal at data expansion, the CPU 7 is considers that proper firmware was transferred, and the firmware stored in the RAM 9 is written in the reloadable nonvolatile memory such as EEPROM 6.