  The physical process of Higgs decaying to two photons is the most important
mode in the discovery of the Higgs particle. Theoretical calculation of
$H\rightarrow\gamma\gamma$ amplitude therefore provides us with an avenue to
explore the underlying theory. Yet some confusion has arisen over the gauge
invariant property of $H\rightarrow\gamma\gamma$ in the standard model. In this
paper, we show that if dimensional regularization is adopted, the on-shell
1-loop $H\rightarrow\gamma\gamma$ amplitude calculated in the standard model is
gauge invariant. In particular, we prove that the physical amplitude calculated
in the unitary gauge is the same as that calculated in the Feynman gauge.
