  We study effects of charged Higgs boson exchange in the $B$ semileptonic
decays $\bar B \rightarrow D^{(*)}\tau\bar\nu_\tau$. Both branching ratio and
$\tau$ polarization are examined. We use the recent experimental data on
semileptonic $B$ decays and the heavy quark effective theory in order to reduce
theoretical uncertainty in the hadronic form factors. Theoretical uncertainty
in the branching ratio is found to be rather small and that in the $\tau$
polarization is almost negligible. Their measurements will give nontrivial
constraints on the charged Higgs sector.
