  In order to investigate charge ordering phenomena under electric field,
static nonequilibrium Hartree approximation (SNHA) method is formulated on the
basis of the nonequilibrium Green's functions introduced by Keldysh. By
applying the SNHA to the 3/4-filling extended Hubbard model on anisotropic
triangular lattice, we study the stabilities and amplitudes of 3-fold and
horizontal charge orders in ${\theta}$ and ${\theta}_d$-(BEDT-TTF)$_2X$ salts
under the electric field. The obtained results show that the electric field
stabilizes the 3-fold state in comparison to the horizontal state. The
amplitude of the 3-fold state tends to decrease by the field, whereas that of
the horizontal state does not change.
