An experimental method to characterize polymer film with various thicknesses, from μm down to nm, in liquid was proposed on the basis of optical reflectivity. Here, poly (methyl methacrylate) (PMMA) and methanol are used as a polymer and liquid, respectively. When a PMMA film is contacted with methanol, the film became thicker with increasing time. Assuming that the swelling behavior of the film can be expressed by Case II mechanism, diffusion coefficients of methanol into the PMMA film were successfully extracted. We observed two diffusion coefficients, fast and slow, assignable to the outermost layer and the remaining internal region including the substrate interface, respectively. Once the film thickness fell short of a threshold value, the diffusion coefficients decreased with decreasing thickness due to the contribution of the depressed segmental mobility at the substrate interface.