  As in the case of the other gauge field theories, there is so called
``gauge'' also in general relativity. This ``gauge'' is unphysical degree of
freedom. There are two kinds of ``gauges'' in general relativity. These are
called the first- and the second-kind of gauges, respectively. The gauge of the
first kind is just coordinate system on a single manifold. On the other hand,
the gauge of the second kind arises in the general relativistic perturbations.
Through the precise distinction of these two concepts of ``gauges'', we develop
second-order gauge-invariant general relativistic perturbation theory.
