PURPOSE: To read magnetic recording by a mangetic detecting device so as to eliminate various external influences and prevent reproduction and faking by writing information on the surface of a rare earth magnet with carbon or a carbon mixture, and applying designated heat treatment to the information to be cooled.

CONSTITUTION: A rare earth magnet pressed plate-like, key-like or bar-like, and sintered, and figures and characters are written on the surface thereof with carbon or a carbon mixture. Subsequently, the magnet is heated at 400-1000°C for one to two hours in a vacuum or in an atmosphere of argon gas. The magnet is fitted to a supporting part 4 made of resin, metal or the like, and the figures or characters are detected by a magnetic detecting device provided on a door to open the door. As the polarization is high holding power, a strong magnetic field of polarization is needed so that the detection by naked- eye observation is generally impossible. Concerning a lock device, locking is performed by a device where a magnetic reading part and a mechanically and electrically locking part are combined in the same manner as the conventional card lock system. Thus, the safety and reliability can be added.