PROBLEM TO BE SOLVED: To provide a preventive and/or therapeutic agent for Acanthamoeba keratitis which has not only a disinfection effect to Acanthamoeba but also a safety to ocular tissues and together with a flexibility of applied contact lens.SOLUTION: A preventive or therapeutic agent of Acanthamoeba keratitis contains an organic nitrogen disinfectant which is a biguanide disinfectant or a quaternary ammonium disinfectant, and a polymer containing a diallyl dialkyl ammonium with a molecular weight of 1000-150000.