The sound velocity ν and sound attenuation coefficient α_s of fluid Hg have been measured at 20 MHz in wide temperature and pressure ranges including the supercritical conditions. In the liquid state the density dependence of v exhibits a clear inflection near 9 g/cm³, where the metal-nonmetal transition occurs, and in the gaseous state ν shows the ideal gas behavior. Near the critical point a minimum of ν is clearly observed and α_s becomes very large. When the sound attenuation per wave length for Hg is compared with that for Ar at constant reduced temperatures and reduced densities, the former is found to be much larger than the latter.