We have studied thermal expansion and contraction of polystyrene thin film supported on silicon substrate using X-ray reflectivity and inelastic neutron scattering techniques. In annealing experiments, we found fast and very slow contraction processes in the thin films above the glass transition temperature. The fast one was attributed to normal relaxation (annealing) process observed in bulk and responsible partly for the reported apparent negative expansivity. In addition, we found extremely slow re-expansion process in the glassy state. The ultra-slow contraction and re-expansion have been assigned to the lateral expansion and contraction of the film, respectively. Using well-annealed films, we have evaluated thermal expansivity as a function of film thickness, and found that it decreases with film thickness in a thickness range below twice of radius of gyration of a polymer chain 2R(g), in the glassy state. The decrease has been attributed to hardening of harmonic force constant due to confinement in a thin film. This idea was confirmed in the inelastic neutron scattering measurements. (c) 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.