The spin-boson model was applied to analyze the temperature dependence of excited state dynamics for the cis-trans photoisomerization of the chromophore in bacteriorhodopsin which was obtained by the Fourier transform of the optical absorption spectrum. The results indicate that the model is valid in the short time region less than about 30 fs and that the excited state dynamics in the time region larger than 30 fs is dominated by the non-harmonic slow vibrational motion, which is temperature independent. It is suggested that this specific vibrational motions might reflect the ultrafast cis-trans isomerization of the chromophore as well as conformation change of the protein environment.