  We study thermodynamic evaporation of Schwarzschild-de Sitter black holes in
terms of a low energy perturbation theory. A small black hole which is far from
the cosmological horizon and observers at the spacelike hypersurface where
black hole attraction and expansion of cosmological horizon balance exactly are
considered. In the low energy perturbation, scalar field equations are solved
in both regions of the hypersurface and scattering amplitudes are derived. And
then the desired thermal temperatures from the two horizons are obtained as a
``minimal'' value of the statistical thermal temperature, and the fine-tuning
between amplitudes gives a relation of the two temperatures.
