The energy of each phonon mode is fluctuating around its equilibrium value, and its power spectral density was derived with a quartz specimen which was illuminated by a laser light. Fluctuations of the Brillouin scattered light intensity were measured by means of the photon-counting technique. The power spectral density has been derived to be 8×10 −5 /f per phonon mode provided that phonon-number fluctuations are independent from mode to mode