To provide a silica-stain adhesion inhibitor which can effectively prevent adhesion of silica stain generated in a water system such as a cooling water system, and an adhesion-inhibiting method.

The silica stain adhesion inhibitor comprises a monomer unit having a carboxyl group, a monomer unit having a sulfonic acid group and a monomer unit having an N-vinyl carboxylic acid amide group in the water system, wherein the adhesion inhibitor includes a copolymer having a composition ratio of the monomer unit having the N-vinyl carboxylic acid amide group in the copolymer of 5 to 30 mole %. The silica-stain adhesion-inhibiting method includes the step of adding the inhibitor.