PURPOSE: To inexpensively and certainly prevent forgery or alteration by providing a heat treatment part to which DC magnetization or signal magnetization is applied to at least a part of a magnetic layer and providing heat treatment marks changed in magnetic characteristics by heating to the heat treatment part at a predetermined interval.

CONSTITUTION: A plurality of heat treatment marks 6 made different in magnetic characteristics from the circumferential magnetic layer by heating are provided to a heat treatment part 5. By this method, the heat treatment part 5 is uniformly magnetized and the magnetism thereof becomes discontinuous. The arranging pattern of the heat treatment marks 6 can be determined at every kind of a magnetic card l. Since the parts free from the heat treatment marks 6 are uniformly magnetized, no output is confirmed and, since magnetism is discontinuous at the positions of the heat treatment marks 6, output pulses are generated. By detecting the output pulses, the positions of the heat treatment marks 6 are investigated to read the arranging patterns of the marks 6 and the kind or quality of the magnetic card l can be discriminated.