  The non-decoupling effects of heavy Higgs bosons on the process $e^+e^- \to
W^+_LW^-_L$ are discussed in the two Higgs-doublet model. The one-loop
corrections to the cross section are calculated by using the equivalence
theorem and the explicit expressions of its deviation from the standard model
are derived. The leading mass contributions to the deviation in expansion by
$s$ are related to the $T$ parameter by the low-energy theorem. The next-to
leading ones are free from the present data, which should be determined by
future experiments. The deviation can amount to $\sim 3$ % at $\sqrt{s} = 1$
TeV under the constraint from the present data, so that it may give useful
information on the Higgs sector in cooperation with data from future linear
colliders.
