  We study the long-distance contribution to $B_s\to\mu^+\mu^-$ decay, which is
generated by the two-photon intermediate state via
$B_s\to\gamma^*\gamma^*\to\mu^+\mu^-$ transition. It is found that the
dispersive two-photon amplitude can interfere with the dominant short-distance
amplitude, which gives rise to new theoretical uncertainty in the branching
ratio of $B_s\to\mu^+\mu^-$. Our analysis shows that, by taking into account
present experimental constraints, this uncertainty could be up to the same
order of magnitude as some theoretical uncertainties of ${\cal
B}(B_s\to\mu^+\mu^-)$ given in the past literature. Future precise studies of
the double radiative $B_s\to\gamma\gamma$ decay, both experimentally and
theoretically, may help to reduce the uncertainty. This novel effect has never
been examined in $B_s\to\mu^+\mu^-$ decay.
