In order to determine the Debye parameter for the surface layer of Si crystal, we have measured the rocking curves of reflection high-energy electron diffraction (RHEED) from a Si(111)-7x7 surface at temperatures extending over a wide range, 293-1073 K. From the analysis of the dynamical calculation of RHEED intensity taking account of the thermal diffuse scattering, the adatoms and rest atoms largely shifted upward compared with the atomic positions derived by Brommer et al. using ab initio molecular-dynamics scheme [Phys. Rev. Lett. 68, 1355 (1992)] and the optimum Debye temperature has been estimated at 420 K, which is much lower than that of bulk Si (505-658 K). Since the Debye temperature is considered to be an averaged value in a few surface layers, the difference means that the vibrational amplitude of surface atom is large compared to bulk atom.