  The propagation of a shock wave into an interstellar medium is investigated
by two-dimensional numerical hydrodynamic calculation with cooling, heating and
thermal conduction. We present results of the high-resolution two-dimensional
calculations to follow the fragmentation due to the thermal instability in a
shock-compressed layer. We find that geometrically thin cooling layer behind
the shock front fragments into small cloudlets. The cloudlets have supersonic
velocity dispersion in the warm neutral medium in which the fragments are
embedded as cold condensations. The fragments tend to coalesce and become
larger clouds.
