  We investigate thermodynamics of hadrons using the Gaussian functional method
(GFM) at finite temperature. Since the interaction among mesons is very large,
we take into account fluctuations of mesons around their mean field values
using the GFM. We obtain the ground state energy by solving the Schr\"{o}dinger
equation. The meson masses are obtained using the energy minimization
condition. The resulting mass of the Nambu-Goldstone boson is not zero even in
the spontaneous chiral symmetry broken phase due to the non-perturbative
effect. We consider then the bound state of mesons using the Bethe-Salpeter
equation and show that the Nambu-Goldstone theorem is recovered. We investigate
further the behavior of the meson masses and the mean filed value as functions
of temperature for the cases of chiral limit and explicit chiral symmetry
breaking.
