A method for inhibiting polymerization of a maleimide, which comprises adding to said maleimide, as a polymerization inhibitor, at least one compound selected from the group consisting of thiodipropionic esters, 4-t-butyl-catechol, 2,6-di-t-butyl-p-cresol,2,5-di-t-butyl-hydroquinone, 2-t-butyl-hydroquinone, 2,4-bis-(n-octylthio)-6-(4-hydroxy-3,5-di-t-butylanilino)-1,3,5-trizaine, 2,2'-thiobis(4-methyl-6-t-butylphenol), 4,4'-thiobis-16-t-butyl-m-cresol), and triethyleneglycol-bis-[3-(3-t-butyl-5-methyl-4-hydroxyphenyl) propionate].