A polystyrene melt, whose molecular weight (weight-average molecular weight = 1050) is about that estimated for a Rouse segment;, has been studied with depolarized photon-correlation and viscosity measurements. Theoretically the dynamic depolarized light scattering is shown to be affected by the reorientational motion of the whole polymer molecule in the long-time region and the local segmental motions in the short-time region. The elastic dumbbell model is used to describe the viscoelasticity and reorientational motion of the polymer molecule. Relaxation times [tau] and tau(v) are extracted from the depolarized light scattering and viscosity measurement results, respectively. The ratio tau(v)/[tau], being independent of temperature as expected, is of the order of magnitude predicted from the theoretical analysis. We come to a similar conclusion supporting the above results by analyzing the depolarized photon-correlation results and the viscoelastic relaxation data in the glass-rubber transition zone of entangled polystyrene melts, which are available in literature.