  In the realistic model of cosmic inflation the inflaton potential should be
flat and stable under quantum corrections. It is natural to imagine that there
is some symmetry behind and an idea of the inflaton as a Nambu-Goldstone boson
of spontaneous breaking of some symmetry has been examined. We give a general
formulation of this idea using the non-linear realization of Nambu-Goldstone
boson in low-energy effective theory with some explicit symmetry breaking to
generate non-trivial potential. The potential is naturally a simple function,
typically the mass term of inflaton, and the scenario of "warm inflation"
should necessarily be applied under the present observational constraints. We
investigate the generation mechanism of necessary thermal dissipation term in
inflaton field equation for "warm inflation" without large thermal corrections
to inflaton potential. A simple numerical analysis is given to investigate the
viability of this scenario.
