  We evaluate the expected measurement accuracy of the branching ratio of the
Standard Model Higgs boson decaying into tau lepton pairs $h \to \tau ^+ \tau
^-$ at the ILC with a center-of-mass energy of $\sqrt{s} = 500$ GeV with a full
simulation of the ILD detector. We assume a Higgs mass of $M_h = 125$ GeV, a
branching ratio of $\mathrm{BR}(h \to \tau ^+ \tau ^-) = 6.32 \%$, beam
polarizations of $P(e^+, e^-) = (-0.8,+0.3)$, and an integrated luminosity of
$\int L dt = 500 \mathrm{fb^{-1}}$. The Higgs-strahlung process $e^+ e^- \to
Zh$ with $Z \to q \overline{q}$ and the $WW$-fusion process $e^+ e^- \to \nu
\overline{\nu} h$ are expected to be the most sensitive channels at $\sqrt{s} =
500$ GeV. Using a multivariate analysis technique, we estimate the expected
relative measurement accuracy of the branching ratio $\Delta(\sigma \cdot
\mathrm{BR}) / (\sigma \cdot \mathrm{BR})$ to be 4.7% and 7.4% for the $q
\overline{q} h$ and $\nu \overline{\nu} h$ final states, respectively. The
results are cross-checked using a cut-based analysis.
