  The radiative corrections to the decay processes of the neutral ($CP$-even)
Higgs boson ($H$) into a longitudinal gauge boson pair, {\it i.e.}, $H
\rightarrow Z_{L}Z_{L}$ and $H \rightarrow W_{L}^{+}W_{L}^{-}$ are analyzed in
the two-Higgs doublet model by assuming that all of the Higgs boson masses are
much greater than the $W$ and $Z$ bosons'. These calculations are motivated to
see if one could see potentially large virtual effects to these decay rates due
to the charged and $CP$-odd neutral Higgs boson masses ($m_{G}$ and $m_{A}$,
respectively) which are supposed to be larger than $m_{H}$. It is pointed out
that, although the radiative corrections to the decay width $\Gamma
(H\rightarrow W_{L}^{+}W_{L}^{-})$ depend sensitively in general on $m_{G}$ and
$m_{A}$, there occurs a screening effect, {\it i.e.,} cancellation in leading
terms once we set $m_{G}=m_{A}$, so that the radiative corrections tend to be
minimized. It is also pointed out that the decay rate $\Gamma (H\rightarrow
Z_{L}Z_{L})$ is fairly insensitive to the other heavier Higgs masses and is
possibly a good measuring tool of the Higgs mixing angle. The mechanism of
these screening phenomena in the Higgs decays is explained on the basis of a
new screening theorem, which we postulate with reference to the custodial
symmetry in the Higgs potential.
