By using near-field scanning optical microscopy (NSOM), patterning of pure refractive-index contrast on a transparent polymer film has been demonstrated. A thin film of poly(methyl methacrylate) doped with 3-phenyl-2,5-norbornadiene-2-carboxylic acid is irradiated with 325 nm UV light from an NSOM probe, resulting in a refractive-index decrease without changes in complete transparency in the visible region and surface flatness. The transmission NSOM image was obtained by the same probe at an incident wavelength of 442 nm. The relative change of the transmission signal is in agreement with the value deduced from a simple model based on the Rayleigh scattering theory of dielectric media.