  We have investigated the variation of induced ferroelectric polarization
under magnetic field with various directions and magnitudes in a staggered
antiferromagnet Ba$_2$CoGe$_2$O$_7$. While the ferroelectric polarization
cannot be explained by the well-accepted spin current model nor exchange
striction mechanism, we have shown that it is induced by the spin-dependent
$p$-$d$ hybridization between the transition-metal (Co) and ligand (O) via the
spin-orbit interaction. On the basis of the correspondence between the
direction of electric polarization and the magnetic state, we have also
demonstrated the electrical control of the magnetization direction.
