“A high-molecular compound or high-molecular antioxidant obtained by reacting at least three compounds of (A) a sulfur-containing organic compound having at least two active hydrogen atoms, (B) a polyisocyanate compound having at least two isocyanate groups and (C) an amine compound having at least two active hydrogen atoms and then cationizing with an acid at least some of amino groups in a resultant reaction product” or “a high-molecular antioxidant obtained by reacting at least three compounds of (A) a sulfur-containing organic compound having at least two active hydrogen atoms, (B) a polyisocyanate compound having at least two isocyanate groups and (E) a nonionic polyol compound having at least two active hydrogen atoms” and a recording medium obtained by using the high-molecular antioxidant. The present invention provides a recording medium, which effectively prevents discoloration or color fading of images by acidic gas in the atmosphere and light and also migration of the images at high temperature and high humidity, and therefore, is suited for the long-term storage of the images.