  The behavior of scalar perturbations on superhorizon scales during the
reheating stage is investigated by replacing the rapidly oscillating inflaton
field by a perfect fluid obtained by spacetime averaging and the WKB
approximation. The influence of the energy transfer from the inflaton to
radiation on the evolution of the Bardeen parameter is examined for realistic
reheating processes. It is shown that the entropy perturbation generated by the
energy transfer is negligibly small, and therefore the Bardeen parameter is
conserved in a good accuracy during reheating. This justifies the conventional
prescription relating the amplitudes of quantum fluctuations during inflation
and those of adiabatic perturbations at horizon crossing in the post-Friedmann
stage.
