Optical absorption spectra were measured on samples of Au and Ag supported on amorphous SiO2 substrates that had a great variation in the state of metal dispersion. The degree of metal dispersion was evaluated with a packing factor determined by transmission electron microscopy. It was found that the spectral peak wavelength (lambda(p)) red-shifts with an increase of the packing factor (q) for either Au or Ag. The observed lambda(p) - q) relation was shown to be well simulated with a simple model based on the Maxwell Garnett theory.