The problem of the invention is to find a polymer mixture which forms an inter-polymer complex by being responsive to temperature even under neutral to alkaline conditions. According to the invention, a temperature responsive inter-polymer complex capable of showing thermal responsiveness in an aqueous solution, which contains a poly-N-acetylacrylamide or polyvinyl alcohol and polyethylene glycol, polyacrylamide or polymethacylamide, is applied to separating agents, immobilized enzymes, denatured protein modifiers, separation method or concentration of microorganisms, purification or concentration of nucleic acids, drug-releasing microcapsules and the like.