  Quantum field theory is constructed upon the assumption of stabilities of the
vacuum and of the one-particle state. For finite temperature, the one-particle
state becomes unstable because of thermal fluctuations, whereas the thermal
vacuum is still stable. In non-equilibrium situation, both the vacuum and the
one-particle state lose their stability. Proposed is the introduction of the
{\it reference vacuum} which takes care of thermal and non-adiabatic
time-evolution of a system, and produces a time-dependent Fock's representation
space. This may provide us with an extension of the concept of {\it dynamical
mapping} where a migration among unitary inequivalent representation spaces can
be handled for non-equilibrium and dissipative systems.
