  We numerically solve the (3+1)-dimensional relativistic hydrodynamical
equation with a bag-model equation of state, which is one of the simplest
models for the first order phase transition. Based on the numerical solution,
we discuss the space-time evolution of the hot fluid produced in ultra
relativistic nuclear collisions. Especially, the space-time structure of the
two-phase co-existing region during the quark-gluon plasma-hadron phase
transition is analyzed in detail. Finally, we investigate the change of the
mass spectra of $J/\psi$, ${\eta}_{c}$ and ${\phi}$ as possible experimental
signatures of the phase transition.
