The present invention is to provide a silver-based inorganic antimicrobial agent that has excellent heat resistance and chemical resistance, that gives little resin coloration, and that has excellent processability. It has been found that the object can be attained by a silver ion-containing zirconium phosphate represented by Formula (1) below by limiting the production conditions to wet synthesis, and the present invention has thus been accomplished. Ag.sub.a M.sub.bZr.sub.c(PO.sub.4).sub.3.nH.sub.2O (1) In Formula (1), M is at least one type of ion selected from an alkali metal ion, a hydrogen ion, and an ammonium ion, a, b, and c are positive numbers and satisfy 1.5.ltoreq.c.ltoreq.2 and a+b+4c=9, and n is no greater than 2.