  The QCD correction of $O(\alpha_s^2)$ to the decay of the Standard Model
Higgs boson into two photons is presented. We consider the contribution coming
from diagrams with a heavy top quark as virtual particle. The first three terms
of the expansion in the inverse top mass is calculated. Expressing the result
through the on-shell top mass M_t, we find large coefficients for the
power-suppressed terms whereas in the $\overline{MS}$ scheme the coefficients
are tiny.
