  We study the longitudinal polarization of the tau lepton in B -> D tau nu
decay. After discussing possible sensitivities of tau decay modes to the tau
polarization, we examine the effect of charged Higgs boson on the tau
polarization in B -> D tau nu. We find a relation between the decay rate and
the tau polarization, and clarify the role of the tau polarization measurement
in the search for the charged Higgs boson.
