To obtain the subject agent presenting excellent antimicrobial action on short-time contact as a function of the resin itself, with substantially no elution of the antimicrobial substance therein, and excellent in resistance to saline or weathering, by including a polymer crosslinked product obtained by copolymerizing an N-vinylcarboxylic acid amide, etc., in the presence of a crosslinking agent.

This agent is obtained by including a polymer crosslinked product obtained by copolymerizing (A) an N-vinylcarboxylic acid amide pref. of formula I (R1 and R2 are each H or methyl) (e.g. N-vinylacetamide), with (B) a monomer having a quaternary phosphonium salt or quaternary ammonium salt on the side chain and shown pref. by formula II, III (R3 to R5 are each H, an alkyl or the like; A is an alkylene; X is a halogen; R9 to R11 are each an aryl, aralkyl or the like) or the like, [e.g. p-vinylbenzyltri(n-octyl)phosphonium chloride], in the presence of a crosslinking agent. This agent is useful in the field of e.g. liquid absorbency improvers for paper and textile products such as paper diapers and sanitary napkins.