  For the purpose of clarifying a new approach to understanding quantum
entanglement using thermofield dynamics (TFD), entanglement entropies of
non-equilibrium finite-spin systems are examined for both traditional and
extended cases. The extended entanglement entropy, $\hat{S}$, is derived, and
it is found that the conditions for the maximum entangled state can be obtained
through this approach. The capacity of the TFD-based method to distinguish
between states in quantum systems is confirmed.
