  The process of top quark pair production at the ILC with subsequent decays of
one of the top quarks to a charged Higgs boson and b-quark is considered. The
charged Higgs decays to tau leptons whose polarization is the opposite to those
coming from W bosons. This difference is reflected in the energy distributions
of the tau decay products in the top quark rest frame, which can be
reconstructed at the ILC using the recoil mass technique. We present an
analysis including spin correlations, backgrounds, ISR/FSR and beamstrahlung,
and show that a fit of the shape of the pion energy spectrum yields the charged
Higgs boson mass with an accuracy of about 1 GeV.
