In previous papers, we found that the mean square displacement < u(2)> in polystyrene thin films decreased with film thickness and discussed two possible causes for the decrease. One is the hard interface layer between the polymer thin film and the substrate, and the other is the spatial confinement effect of polymer chains. In order to disclose the cause of the decrease of < u(2)> with film thickness, we studied the molecular weight M(w) dependence of < u(2)> in polystyrene thin films. We found that < u(2)> was independent of Mw within experimental error, concluding that the interface effect is a main reason for the decrease of < u(2)> with film thickness.