To provide a method capable of inserting an antimicrobial agent into the polymer structure of a fibrous material without using an extremely high-temperature heat process.

This method comprises inserting an antimicrobial agent into a fibrous material such as a yarn, a yarn woven fabric, a single product of fiber or the like. In this method, the fibrous material is passed through an aqueous bath containing an antimicrobial agent selected from the group consisting of (a) a halogeno-o-hydroxydiphenyl compound, (b) a phenol derivative, (c) benzyl alcohol, (d) chlorohexidine and its derivative, (e) a 12-14C alkylbetaine and an 8-18C amidoalkylbetaine, (f) an amphoteric surfactant, (g) a trihalocarboanilide, (h) a quaternary compound and a polyquaternary compound and (i) a thiazole compound.