PURPOSE: To provide a silk with excellent mildewproofing properties and concentratedly dyeing properties by treating silk yarn with methacrylamide by jointly using an alkyldimethylpropylomethacrylamide ammonium salt.

CONSTITUTION: In subjecting silk to graft processing with methacrylamide without damaging feeling of handling of silk yarn, silk yarn is subjected to graft polymerization by jointly using 2.5-25wt.%, based on methacrylamide, of an alkyldimethylpropylomethacrylamide ammonium halide shown by the formula (R is ≤16C alkyl; X is Cl, Br or I) to give the objective silk subjected to graft polymerization having excellent mildewproofing properties and concentratedly dyeing properties without damaging hand touch feeling of silk.