Provided are a composite of nanoparticles of a metal such as gold, silver, a platinum metal, or copper, and a polymer, the composite allowing formation of a metal film having a sufficiently low resistivity in terms of practicality simply by drying at room temperature without requiring any special heating-baking step or any step of removing a protective agent with a solvent; a dispersion liquid of the composite; methods for producing the foregoing; and a plastic substrate formed from the dispersion liquid. The composite comprises a (meth)acrylic-based copolymer having, as side chains, a polyethylene glycol chain and a phosphate residue represented by —OP(O)(OH)2 and having, at at least one end of the molecular chain, —SR (where R represents an alkyl group or the like).