PURPOSE: To prevent data from being copied by processing specifically to the cyclic redundancy check (CRC) data contained in data.

CONSTITUTION: At the same time when a CRC write timing signals is outputted from a floppy disk control circuit 2, the circuit 2 outputs CRC data, which is supplied to one input terminal of an adder 10 through an AND circuit 8. Said CRC write timing signal is outputted to a shift register 6 at the same time. Consequently, a previously registered cryptographic code is read out of a shift register 6 and supplied to the other input terminal of an adder 10. Therefore, the adder 10 inputs the CRC data and cryptographic code at the same timing and outputs the CRC data with the cryptographic code. This CRC data with the cryptographic code is sent out to a floppy disk device 1 through an AND circuit 22 and an OR circuit 26 and written at the tail of a data field on a floppy disk.