A non-specific adsorption inhibitor includes a water-soluble polymer obtained by polymerizing a monomer unit that includes a monomer (A) that forms a polymer that has a lower critical solution temperature (LCST) of 80° C. or less in an aqueous solution under normal pressure when subjected to homopolymerization, and a monomer (B) that forms a polymer that does not have the LCST in an aqueous solution under normal pressure when subjected to homopolymerization.