Abstract: A surface modifying composition for modifying a surface of a formed product made of high-density polyethylene or ultra-high molecular weight polyethylene, the composition comprising: a copolymer having a unit of a first monomer having an aliphatic group having 10 or more carbon atoms and a unit of a second monomer having any of an amino group, an epoxy group, and an ether group, and a solvent having a boiling point of 100° C. or more and being at least one selected from the group consisting of a halogen-based solvent, an alkane solvent, a cycloalkane solvent, a dicycloalkane solvent, an aromatic solvent, and a nitro-based solvent.