  We study the structure of the time evolution of the density matrix in contact
with a thermal bath in a standard projection operator sheme. The reduced
density matrix of the system in the steady state is obtained by tracing out the
degree of freedom of the thermal bath from the equilibrium density matrix of
the total system. This reduced density matrix is modified by the interaction,
and is different from that of the equilibrium of the system alone. We
explicitly calculate the contribution of each term in quantum master equation
to the realization of the steady state density matrix, and make clear roles of
each term. By making use of the role of each term, the properties of the
commonly used quantum master equation are examined.
