Agent for preventing formation of biofilm, and composition for oral use

Provided are: an agent for preventing the formation of biofilms, the agent improving the sustainability of the effect of a drug without increasing the concentration of the drug or the duration of treatment, sustainably maintaining a good oral environment, and preventing maturation into biofilms; and a composition for oral use comprising the agent.　Specifically, the agent for preventing the formation of biofilms comprises a copolymer including: 10-90mol% of a structural unit (A) having a weight average molecular weight of 10,000-5,000,000 and based on 2-(meth)acryloyloxyethyl phosphorylcholine; and 90-10mol% of at least one structural unit selected from the group consisting of a structural unit (B1) based on an alkyl group-containing (meth)acrylic monomer, a structural unit (B2) based on a quaternary ammonium group-containing (meth)acrylic monomer, and a structural unit (B3) based on a (meth)acrylamide monomer.