  We consider a two-loop contribution of Higgs-mediated penguin diagram to
$B(B_s \to \mu^+\mu^-)$ at large $\tan\beta$ in the MSSM, motivated by a
recently proposed two-loop magnetic penguin diagrams by Chen and Geng [1].
Typically the two-loop diagram is a $\alpha_s$ correction to the one-loop
contributions. However the new contribution can dominate one-loop contributions
in a region of parameter space where $\mu$, $A_t$ and $A_b$ are very large and
scalar top, scalar bottom and charged Higgs are light. Both constructive and
destructive interferences are possible. The total branching ratio can be
drastically changed from the one-loop result.
