The local conformation of polymer chains in a film at the interface with a substrate was studied by sumfrequency generation spectroscopy. When a polystyrene (PS) film was prepared on a quartz substrate by a spin-coating method, the chains were aligned in the interfacial plane of the substrate. This interfacial orientation of chains was not observed for a PS film prepared by a solvent-casting method. The local conformation of chains at the substrate interface was interestingly unchanged even at a temperature that was 80 K higher than the bulk glass transition temperature. This observation indicates that polymer chains at the substrate interface can be only partially relaxed under conditions where the bulk chains are fully relaxed. On the other hand, interfacial chains could be easily relaxed by the solvent annealing.