Disclosed is a novel, simple, general-purpose technique which can be applied for modifying the surface and/or interface of a polymer. A polymer structure whose surface and/or interface is modified can be obtained by mixing a branched polymer (preferably a dendritic polymer, a comb-like polymer or a hyperbranched polymer) into a linear polymer which serves as the matrix polymer, thereby condensingthe branched polymer on the surface and/or interface of the matrix polymer.