Site-selective fluorescence spectroscopy and hole-burning spectroscopy have been performed for Zn-substituted myoglobin (ZnMb) in the red spectral region at low temperatures. The site- energy distribution of the chromophore and the fluorescence spectrum of ZnMb in a single site have been obtained from the analysis of laser-induced fluorescence spectra. It has been found that this single-site fluorescence spectrum can be regarded as the homogeneous spectrum. The comparison of the hole spectra between freeze-dried ZnMb powder and ZnMb in glycerol- water mixture shows that the phonon-sideband profiles of these spectra as well as that of the single-site fluorescence spectrum reflect the density of states of vibrational modes of the protein itself. The density of states of low-frequency vibrational modes of ZnMb weighted by the coupling strength between the electrons of the chromophore and the vibrations of the polypeptide chain has been determined from the above-obtained single-site fluorescence spectrum by solving an integral equation numerically.