Disclosed are: an antibacterial processing agent which is capable of releasing silver ions at a just enough concentration in clean water such as drinking water or circulating water, thereby making the water antibacterial, and which is capable of maintaining the silver releasing ability for a long period of time； and a water treatment method using the antibacterial processing agent. Specifically disclosed is an antibacterial processing agent for a water treatment, which is characterized by being composed of a resin composition that contains a resin having a standard water content of 1-10% by weight and 1-30% by weight of a silver-substituted zirconium phosphate that is represented by formula (1) as an antibacterial agent. AgaMbZrcHfd(PO4)3·nH2O (1) In formula (1), M represents at least one kind of ion selected from the group consisting of an alkali metal ion, an alkaline earth metal ion, an ammonium ion, a hydrogen ion or an oxonium ion； a, b and c each independently represents a positive number； d represents 0 or a positive number； and n represents 0 or a positive number of 2 or less.
