Provided are a water-treating agent which includes a specific nonionic polymer and a phosphorus compound and can exhibit the ability to inhibit the deposition of silica-based scale under wide ranges of water quality conditions and temperature conditions; and a method of treating water. The inhibitor for silica-based scale includes a nonionic polymer and a phosphorus compound, the nonionic polymer being: a polymer formed by the reaction of an unsaturated double bond and including units derived from at least one monomer selected from the group consisting of (meth)acrylamide, N-substituted (meth)acrylamide and N-vinyl lactam; and/or one or more polymers having an ether group and selected from the group consisting of alkyl alcohol/polyalkylene oxide adducts, aliphatic monocarboxylic acid/polyalkylene oxide adducts, and polyvinyl alcohol/polyalkylene oxide adducts.