  Recent blazar observations provide growing evidence for the presence of
magnetic fields in the extragalactic regions. While a natural speculation is to
associate the production to inflationary physics, it has been known that
magnetogenesis solely from inflation is quite challenging. We therefore study a
model in which a non-inflaton field $\chi$ coupled to the electromagnetic field
through its kinetic term, $-I^2(\chi) F^2 /4$, continues to move after
inflation until the completion of reheating. This leads to a post-inflationary
amplification of the electromagnetic field. We compute all the relevant
contributions to the curvature perturbation, including gravitational
interactions, and impose the constraints from the CMB scalar fluctuations on
the strength of magnetic fields. We, for the first time, explicitly verify both
the backreaction and CMB constraints in a simple yet successful magnetogenesis
scenario without invoking a dedicated low-scale inflationary model in the
weak-coupling regime of the kinetic coupling model.
