  We consider the chargino contribution to the CP violation in $B \to \phi K_S$
decay in the minimal supersymmetric standard model at large $\tan\be$. It is
shown that the Wilson coefficient $C_{8g}$ of the chromomagnetic penguin
operator can be significantly enhanced by the chargino-mediated diagrams while
satisfying other direct/indirect experimental constraints. The enhanced
$C_{8g}$ allows large deviation in the CP asymmetry from the standard model
prediction, especially it can explain the apparent anomaly reported by BaBar
and Belle.
