Invisible bar codes such as magnetic ones play an important role in systems requiring security. The magnetic bar code system described in this paper uses magnetic anisotropy as a carrier of information. Three kinds of magnetic ink were used, containing iron base alloy, Mn-Zn ferrite powder, and carbonyl iron powder. Although the level of the output voltage needs subsequent amplification, all of these magnetic ink can be used for making magnetic bar codes, especially the one containing iron base alloy. We explain a possible mechanism for two sub-peaks occurring berfore and after the main peak.