Provided is a biofilm formation-preventing agent that improves the sustainability of an effect of a drug, and keeps an oral environment satisfactory in a sustainable manner, without an increase in drug concentration or treatment time to prevent the maturation of dental plaque into a biofilm, and a composition for oral use containing the biofilm formation-preventing agent. More specifically, the biofilm formation-preventing agent includes a copolymer having a weight-average molecular weight of from 10,000 to 5,000,000 and containing: 10 mol% to 90 mol% of a constituent unit (A) based on 2-(meth)acryloyloxyethyl phosphorylcholine； and 90 mol% to 10 mol% of at least one kind of constituent unit selected from the group consisting of a constituent unit (B1) based on an alkyl group-containing (meth)acrylic monomer, a constituent unit (B2) based on a quaternary ammonium group-containing (meth) acrylic monomer, and a constituent unit (B3) based on a (meth)acrylamide-based monomer.
