To provide a water treatment agent with does not contain an environment contamination substance such as a heavy metal and phosphorous, and has an excellent corrosion suppression effect and scale suppression effect of metal in a water system, and a water treatment method.

The water treatment agent is characterized by containing, as an effective component, one kind or more of polymers polymerized in an aqueous solvent which is selected from: (A) a maleic polymer polymerized in an aromatic solvent whose light absorption coefficient at 190 nm is 0.09 to 0.2(dm³/mg.cm); and (B) an itaconic polymer, a maleic polymer, and a copolymer of an intaconic acid and a maleic acid, and also characterized by simultaneously using the (A) component and the (B) component.