  We study the pairing mechanism in iron pnictide superconductors based on the
five-orbital Hubbard-Holstein model. Due to Fe-ion oscillations, the s-wave
superconducting (SC) state without sign reversal (s_{++}-wave state) is induced
by orbital fluctuations by using realistic model parameters. The virtue of the
present theory is that the famous empirical relation between Tc and the
As-Fe-As bond angle is automatically explained, since the electron-phonon
(e-ph) coupling that creates the orbital fluctuations is the strongest when the
As$_4$-tetrahedron is regular. The negative iron isotope effect is also
reproduced. In addition, the magnitude of the SC gap on hole-pockets is
predicted to be rather insensitive to the corresponding d-orbital (xz/yz- or
z^2-orbital), which is consistent with the recent bulk-sensitive angle-resolved
photoemission spectroscopy (ARPES) measurement for (Ba,K)Fe$_2$As$_2$ and
BaFe$_2$(As,P)$_2$. These obtained results indicate that the
orbital-fluctuation-mediated s_{++}-wave state is a plausible candidate for
iron pnictides.
