  We study superconducting instability from orbital nematic fluctuations in a
minimal model consisting of the $d_{xz}$ and $d_{yz}$ orbitals, and choose
model parameters which capture the typical Fermi surface geometry observed in
iron-based superconductors. We solve the Eliashberg equations down to low
temperatures with keeping the renormalization function and a full momentum
dependence of the pairing gap. When superconductivity occurs in the tetragonal
phase, we find that the pairing gap exhibits a weak momentum dependence over
the Fermi surfaces. The superconducting instability occurs also inside the
nematic phase. When the $d_{xz}$ orbital is occupied more than the $d_{yz}$
orbital in the nematic phase, a larger (smaller) gap is realized on the
Fermi-surface parts, where the $d_{xz}$ ($d_{yz}$) orbital component is
dominant, leading to a substantial momentum dependence of the pairing gap on
the hole Fermi surfaces. On the other hand, the momentum dependence of the gap
is weak on the electron Fermi surfaces. We also find that while the leading
instability is the so-called $s_{++}$-wave symmetry, the second leading one is
$d_{x^2-y^2}$-wave symmetry. In particular, these two states are nearly
degenerate in the tetragonal phase whereas such quasi-degeneracy is lifted in
the nematic phase and the $d_{x^2-y^2}$-wave symmetry changes to highly
anisotropic $s$-wave symmetry.
