  Fundamental formulae representing the density perturbation spectrums
generated in inflationary scenarios are rigorously proved. Quantum fluctuations
as initial conditions for structure generation in some inflationary era can be
calculated in general scale. Thus, we could avoid the matching of the large and
the small scale results at the horizon crossing epoch. This is possible because
the perturbed scalar field equation in a special gauge has some remarkable nice
properties. In the uniform-curvature gauge the contributions from the metric
fluctuations effectively disappear in certain expansion stages, and also we can
derive a simple general solution in the large scale limit.
