  There are several works searching for a clue of trans-Planckian physics on
the primordial density perturbation spectrum. Here we would like to point out
an important aspect which has been overlooked so far. When we consider a model
in which the primordial density perturbation spectrum is modified due to
trans-Planckian physics, the energy density of fluctuations of the inflaton
field necessarily becomes significantly large, and hence its back reaction to
the cosmic expansion rate cannot be neglected.
