A method for producing an organic polymer thin membrane, the method comprising the steps of: preparing a solution of at least one compound selected from the group consisting of a compound having a hydrophilic functional group, a compound having a polyalkylene glycol chain within the molecule, and a compound having a functional group selected from a hydroxyl group and a carboxyl group; forming a thin membrane of the solution on a base material surface; and forming an organic polymer thin membrane through a polymerization reaction caused by irradiating the exposed surface of the thin membrane with a plasma or an electron beam, such that the composition is homogeneous along the plane direction of the thin membrane, and the chemical composition gradually varies in the cross sectional direction perpendicular to the plane direction of the thin membrane, wherein the gradual change in the chemical composition of the thin membrane is confirmed by using at least one selected from: transmission electron microscopy of a cross section across the thin membrane from the exposed surface to the back surface after osmium staining of the thin membrane; energy dispersive X-ray inner composition analysis of the thin membrane from the exposed surface to the back surface; atomic force microscopy of surface structures on both surfaces of the thin membrane; and glazing-incidence small angle X-ray scattering orientation analysis of both thin membrane surfaces. preparing a solution of at least one compound selected from the group consisting of a compound having a hydrophilic functional group, a compound having a polyalkylene glycol chain within the molecule, and a compound having a functional group selected from a hydroxyl group and a carboxyl group; forming a thin membrane of the solution on a base material surface; and forming an organic polymer thin membrane through a polymerization reaction caused by irradiating the exposed surface of the thin membrane with a plasma or an electron beam, such that the composition is homogeneous along the plane direction of the thin membrane, and the chemical composition gradually varies in the cross sectional direction perpendicular to the plane direction of the thin membrane, wherein the gradual change in the chemical composition of the thin membrane is confirmed by using at least one selected from: transmission electron microscopy of a cross section across the thin membrane from the exposed surface to the back surface after osmium staining of the thin membrane; energy dispersive X-ray inner composition analysis of the thin membrane from the exposed surface to the back surface; atomic force microscopy of surface structures on both surfaces of the thin membrane; and glazing-incidence small angle X-ray scattering orientation analysis of both thin membrane surfaces. transmission electron microscopy of a cross section across the thin membrane from the exposed surface to the back surface after osmium staining of the thin membrane; energy dispersive X-ray inner composition analysis of the thin membrane from the exposed surface to the back surface; atomic force microscopy of surface structures on both surfaces of the thin membrane; and glazing-incidence small angle X-ray scattering orientation analysis of both thin membrane surfaces.