  The origin of the electronic nematicity and its remarkable
material-dependence are famous longstanding unsolved issues in Fe-based
superconductors. To attack these issues, we focus on the in-plane anisotropy of
the resistivity: In the nematic state in FeSe, the relation $\rho_x>\rho_y$
holds, where $\rho_{x(y)}$ is the resistivity along the longer (shorter) Fe-Fe
axis. In contrast, the opposite anisotropy $\rho_x<\rho_y$ is realized in other
undoped Fe-based superconductors. Such nontrivial material dependence is
naturally explained in terms of the strongly orbital-dependent inelastic
quasiparticle scattering realized in the orbital-ordered state. The opposite
anisotropy between FeSe ($\rho_x>\rho_y$) and other undoped compounds
($\rho_x<\rho_y$) reflects the difference in the number of hole-pockets. We
also explain the large in-plane anisotropy of the thermoelectric power in the
nematic state.
