  In the investigation of the evolution of cosmological perturbations in
inflationary universe models the behavior of perturbations during the reheating
stage is the most unclear point. In particular in the early reheating phase in
which a rapidly oscillating scalar field dominates the energy density, the
behavior of perturbations are not known well because their evolution equation
expressed in terms of the curvature perturbation becomes singular. In this
paper it is shown that in spite of this singular behavior of the evolution
equation the Bardeen parameter stays constant in a good accuracy during this
stage for superhorizon-scale perturbations except for a sequence of negligibly
short intervals around the zero points of the time derivative of the scalar
field. This justifies the conventional formula relating the amplitudes of
quantum fluctuations during inflation and those of adiabatic perturbations at
horizon crossing in the Friedmann stage, except for possible corrections
produced by the energy transfer from the scalar field to radiation in the late
stage of reheating. It is further shown that outside the above sequence of time
intervals the behavior of the perturbations coincides in a good accuracy with
that for a perfect fluid system obtained from the original scalar field system
by the WKB approximation and a spacetime averaging over a Hubble horizon scale.
