  We present a new formulation for the evaluation of the primordial spectrum of
curvature perturbations generated during inflation, using the fact that the
Wronskian of the scalar field perturbation equation is constant. In the
literature, there are many works on the same issue focusing on a few specific
aspects or effects. Here we deal with the general multi-component scalar field,
and show that our new formalism gives a method to evaluate the final amplitude
of the curvature perturbation systematically and economically. The advantage of
the new method is that one only has to solve a single mode of the scalar field
perturbation equation backward in time from the end of inflation to the stage
at which the perturbation is within the Hubble horizon, at which the initial
values of the scalar field perturbations are given. We also clarify the
relation of the new method with the new delta N formalism recently developed in
Lee et al.(2005).
