To obtain a color fading preventing agent, excellent in the ability of dyed cellulosic fibers to prevent fading with a nitrogen oxide gas by partially hydrolyzing an N-vinylformamide-based polymer.

An aqueous solution of sodium hydroxide is added to an aqueous solution of an N-vinylformamide-based polymer and partial hydrolytic treatment of the polymer is carried out at a temperature within the range of 50-110°C for 1-8 hr under basic conditions to afford a fading preventing agent containing 2-90 mol%, preferably 10-60 mol% structural unit represented by [CH2-CH (NHCOR)] (R is H or a 1-4C alkyl) and 10-98 mol%, preferably 40-90 mol% structural unit represented by [CH2-CH(NH2)].