Transparent polymer film was fluorinated by contacting with fluorine gas. The fluorinated layer depth was determined from the visual spectrum. The relation between fluorinated depth d and reaction time t was represented by d = k·t½. This result indicates that fluorine diffusion plays a predominant role in forming the diffusion profile. The apparent activation energy of fluorine gas diffusion into polystyrene (PS) and polycarbonate (PC) was 2.8 kcal/mole and 6.6 kcal/mole, respectively. The refractive index distribution from surface to center, which corresponds to the fluorine content in the layer, was determined by interference microscopy. PS and PC showed nearly stepwise change of refractive index. On the other hand, poly(methyl methacrylate) showed a gradual change. These results are compatible with fluorine analysis data and IR spectra of the fluorinated polymers. On the basis of these results, antireflecting layers on plastic lens or plate, plastic optical fiber, or fiber sheet were formed.