The present invention is a method which erase information printed on a recording medium if necessary and thus reuse the recording medium, and includes 1) decolorable ink compositions which are not erased during the use but erased by a dye readily decolorable by a decolorant, 2) a decolorant composition including a thermal initiator or photoinitiator, and 3) a method for coating the decolorant and applying heat or irradiating UV to erase the dye. According to the present invention, it is possible to prepare various writing inks, and particularly, when applied to recording medium with a magnetic stripe as well as the general purpose papers, it is possible to reduce the production cost significantly, obtain a large import substitution effect of the magnetic ink all of which are imported, and largely reduce the environmental pollution.