  We investigate the process of the Schwinger effect by axial coupling in the
natural single-field inflation model in two parts. First we consider the
Schwinger effect when the conformal invariance of Maxwell action should be
broken by axial coupling $ I(\phi)F_{\mu\nu}\tilde{F}^{\mu\nu} $ with the
inflaton field by identifying the standard horizon scale $ k=aH $ at the very
beginning of inflation for additional boundary term and use several values of
coupling constant $ \chi_{1} $ and estimate electric and magnetic energy
densities and energy density of produced charged particles due to the Schwinger
effect.We find that for both coupling functions the energy density of the
produced charged particles due to the Schwinger effect is so high and spoils
inflaton field.In fact the strong coupling or back-reaction occurs because the
energy density of produced charged particles is exceeding of inflaton field.We
use two coupling functions to break conformal invariance of maxwell action.The
simplest coupling function $ I\left(\phi\right)=\chi_{1}\frac{\phi}{M_{p}} $
and a curvature based coupling function $ I\left(\phi\right)=
12\chi_{1}e^{\left(\sqrt{\frac{2}{3}}\frac{\phi}{M_{p}}\right)}\left[\frac{1}{3M_{p}^{2}}\left(4V\left(\phi\right)\right)+\frac{\sqrt{2}}{\sqrt{3}M_{p}}\left(\frac{dV}{d\phi}\right)\right]
$ where $V\left(\phi\right) $ is the potential of natural inflation. In second
part , in oder to avoid strong back-reaction problem we identify the horizon
scale $ k_{H}=aH|\zeta| ,
\zeta=\frac{{I}^{\prime}\left(\phi\right)\dot{\phi}}{H} $ in which a given
Fourier begins to become tachyonically unstable.The effect of this scale is
reducing the value of coupling constant $ \chi_{1} $ and weakening the
back-reaction problem but in both cases strong coupling or strong back-reaction
exists and the Schwinger effect is impossible.
