PURPOSE: To improve a forgery preventive effect by giving information to a magnetic bar code using the magnetic material of low coercive force not detected by a magnet viewer, etc., and reading a magnetic pattern from it as changing voltage.

CONSTITUTION: The magnetic material which is the low coercive force magnetic material of an extent not detected by the magnet viewer and is saturated by an external magnetic field below 5000Oe is used as the magnetic material of the magnetic bar code 23. The magnetic material which is the low coercive force magnetic material not detected by the magnet viewer and in addition, is saturated by the external magnetic field over 5000Oe is used as the magnetic material for the magnetic bar code 24. In the case that the recorded information is read by flowing a DC bias current in a reading magnetic head, the bar codes 23, 24, 25 are read as changing slice level voltage so as to increase it successively, and the truth or the falseness of the recorded information is judged by these different output signal patterns. Thus, the forgery preventive effect can be remarkably improved.