  The current status of the recent developments of the second-order
gauge-invariant cosmological perturbation theory is reviewed. To show the
essence of this perturbation theory, we concentrate only on the universe filled
with a single scalar field. Through this review, we point out the problems
which should be clarified for the further theoretical sophistication of this
perturbation theory. We also expect that this theoretical sophistication will
be also useful to discuss the theoretical predictions of Non-Gaussianity in CMB
and comparison with observations.
