An antimicrobial agent utilizes a resin having acrylonitrile and styrene and/or a conjugated diene as units. The antimicrobial agent includes a metal-salt-containing copolymer, and the metal-salt-containing copolymer is obtained by treating a copolymer containing acrylonitrile and at least one of styrene and a conjugated diene as units with an acid to thereby introduce acid radicals into the copolymer, and converting at least part of the acid radicals into a salt of at least one metal selected from among Ag, Cu, and Zn.