Surface mobility in polystyrene (PS) films was studied using (PS/deuterated PS) bilayer films, which were prepared by attaching original two surfaces together. Time evolution of the bilayer interface at various temperatures was examined by dynamic secondary ion mass spectroscopy in conjunction with neutron reflectivity. When the bilayer was annealed at a temperature above bulk glass transition temperature, T-g(b), the interfacial thickening was well expressed by the context of Fickian diffusion. On the other hand, in the case of an annealing temperature above surface glass transition temperature, T-g(s), and below T-g(b), the interface monotonically thickened with the time at first and then turned to be independent. This means that chains went across the "mobile" interface and then reached the "dead" bulk region in which the diffusivity should be frozen. Hence, it was claimed that chains could diffuse discernibly in the surface region even at a temperature lower than the T-g(b). On the basis of temperature and molecular weight dependences of quasi-equilibrium interfacial thickness after a sufficiently long time, a possible model of mobility gradient in the surface region was proposed.