The conformation of single polymer chain restricted in a two-dimensional plane was studied by scanning near-field optical microscopy (SNOM). The single dye-labeled chain in a monolayer of poly(isobutyl methacrylate) (PiBMA) with a high molecular weight of ca. 5 X 106 was directly observed by SNOM as a circular shape with a diameter of 200-300 nm. The observed chain size was considerably smaller than that for a self-avoiding random walk chain generated by computer simulation, indicating that the polymer chain takes a contracted conformation in two dimensions. SNOM imaging was carried out for the labeled PiBMA chain dispersed in the matrix monolayer with 1000-times lower molecular weight. The long chain isolated among shorter chains had an expanded form as large as a random conformation. These results were in good agreement with the scaling theory, providing an unambiguous evidence for the first time with the real image for a single chain conformation by SNOM. (c) 2005 Elsevier Ltd. All rights reserved.