  The dynamical density fluctuations around QCD critical point (CP) are
analyzed using relativistic dissipative fluid dynamics, and we show that the
sound mode around the QCD CP is strongly attenuated whereas the thermal
fluctuation stands out there. We speculate that if possible suppression or
disappearance of a Mach cone, which seems to be created by the partonic jets at
RHIC, is observed as the incident energy of the heavyion collisions is
decreased, it can be a signal of the existence of the QCD CP. We have presented
the Israel-Stewart type fluid dynamic equations that are derived rigorously on
the basis of the (dynamical) renormalization group method in the second part of
the talk, which we omit here because of a lack of space.
