Dynamics of polystyrene molecules inside controlled pore glasses, a typical confining geometry, was studied by dynamic light scattering over a wide range of concentrations of polystyrene in solutions in equilibrium with the porous glasses. Index-matching of the solvent to the silica glasses effectively facilitates the acquisition of information on the dynamics of polymer chains inside the pore without compromising that information by multiple light scattering. When the concentration outside the pore is much smaller than the overlap concentration ν*, the apparent diffusion coefficient Dpore of polymers within the pore shows little dependence on concentration. As the outside concentration increases and approaches ν*, Dpore rapidly increases. This tendency is more pronounced for polystyrene samples that have higher molecular weights and are predicted to have a lower concentration inside the pore. With further increases of concentration beyond ν*, Dpore approaches the apparent diffusion coefficient outside the pore. Moreover, Dpore becomes almost the same for the three different molecular weights of polystyrene fractions studied and depends primarily on the weight concentration of the solute outside the pore. These features are typical of a semidilute solution regime for flexible polymers. The rapid increase in Dpore is ascribed to a drastic increase of the polymer concentration inside the pore, which results from an equilibration of the chemical potential of the polymer molecule between the interior of the pore and the exterior. Thus, a rapid increase in the osmotic pressure outside the pore drives the polymers into pore channels even at the expense of reduced entropy. We present a quantitative analysis of this highly nonlinear partitioning of polymer molecules.