  To understand the nematicity in Fe-based superconductors, nontrivial
$k$-dependence of the orbital polarization ($\Delta E_{xz}(k)$, $\Delta
E_{yz}(k)$) in the nematic phase, such as the sign reversal of the orbital
splitting between $\Gamma$- and X,Y-points in FeSe, provides significant
information. To solve this problem, we study the spontaneous symmetry breaking
with respect to the orbital polarization and spin susceptibility
self-consistently. In FeSe, due to the sign-reversing orbital order, the hole-
and electron-pockets are elongated along the $k_y$- and $k_x$-axes
respectively, consistently with experiments. In addition, an electron-pocket
splits into two Dirac cone Fermi pockets with increasing the orbital
polarization. The orbital-order in Fe-based superconductors originates from the
strong positive feedback between the nematic orbital order and spin
susceptibility.
