In order to obtain information about structural relaxations of polymers within a time window of several nanoseconds, the absorption; site-selective steady-state fluorescence and time-resolved fluorescence spectra have been measured for polyvinyl alcohol doped with rhodamine 640 in the 150-300 K temperature range. The temperature dependence of the absorption and fluorescence spectra has been analyzed on the basis of one- and two-dimensional configuration-coordinate models. In spite of the measurement below the glass-transition temperature of the matrix, the existence of a fast relaxation process which is completed within a few hundred ps has been clarified. The magnitude of this relaxation increases with increasing temperature, while the relaxation mechanism cannot be ascribed to the thermal crossing of static energy barriers. It has been found that the experimental results are not explained by the two-dimensional configuration coordinate model in which the fast and slow structural relaxations are assumed to occur independently along the two axes. A relaxation process triggered by temperature-dependent release from the constraint preventing the structural change is shown to account for the experimental results well using a one-dimensional configuration coordinate model.