Charged Higgs Effects on Exclusive Semi-tauonic $\bbox{B}$ Decays

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.