The origin of the unique properties of a polymer material is the large entropic term of a single molecule, which has a chain-like structure with a large molecular weight. From the viewpoint of understanding the fundamental polymer physics, conformation of the single polymer chain is one of the most important matters; however, it has been difficult to examine the behavior of a single chain because of the limitation of conventional experimental methods. Recent developments in optical microscopy allow the fluorescence imaging beyond the diffraction limit of light, and the author's group showed that the conformation and the dynamics of a single polymer chain can be examined by the high-resolution fluorescence imaging. This review presents the application of optical microscopy with nanometric spatial resolution to study the polymer materials at the single-chain level.