  Statistical mechanics for states with complex eigenvalues, which are
described by Gel'fand triplet and represent unstable states like resonances,
are discussed on the basis of principle of equal ${\it a priori}$ probability.
A new entropy corresponding to the freedom for the imaginary eigenvalues
appears in the theory. In equilibriums it induces a new physical observable
which can be identified as a common time scale. It is remarkable that in spaces
with more than 2 dimensions we find out existence of stable and quasi-stable
systems, even though all constituents are unstable. In such systems all
constituents are connected by stationary flows which are generally observable
and then we can say that they are semiclassical systems. Examples for such
semiclassical systems are constructed in parabolic potential barriers. The
flexible structure of the systems is also pointed out.
