With a microorganism-inactivating agent comprising a composition containing γ-polyglutamic acid and/or cross-linked product of γ-polyglutamic acid a calcium ion donor a sodium ion donor and an aluminum ion donor, microorganisms in water are inactivated. By further including one or more selected from microbiostatic agents and microbicides into the microorganism-inactivating agent, various microorganisms in water are inactivated to a level at which the water is suitable for drinking.