  The superconducting (SC) state of iron-based compounds in both tetragonal and
orthorhombic phases is studied on the basis of an effective Hamiltonian
composed of the kinetic energy including the five Fe 3d-orbitals, the
orthorhombic crystalline electric field (CEF) energy, and the two-orbital
Kugel'-Khomski\u{i}-type superexchange interaction. Our basic assumption is
that the antiferromagnetic (AF) state in the parent compounds can be described
by the $d_{xz}$ and $d_{yz}$ orbitals, and that the electrons in these orbitals
have relatively strong electron correlation in the vicinity of the AF state. In
order to study the physical origin of the structure-sensitive SC transition
temperature, the effect of orthorhombic distortion is taken into account as the
energy-splitting, $\Delta_{\textrm{ortho.}}$, between the $d_{xz}$ and $d_{yz}$
orbitals. We find that the eigenvalue of the linearized gap equation decreases
accompanied with the reduction of the partial density of states for the
$d_{xz}$ and $d_{yz}$ orbitals as $\Delta_{\textrm{ortho.}}$ increases, and
that the dominant pairing symmetry is an unconventional fully gapped
$s_{+-}$-wave pairing. We also find large anisotropy of the SC gap function in
the orthorhombic phase. We propose that the CEF energy plays an important role
in controlling $T_{\textrm{c}}$ and the SC gap function, and that
orbital-selective superconductivity is a key feature in iron-based
superconductors, which causes the structure-sensitive $T_{\textrm{c}}$.
