  A common idea for the origin of the Galactic diffuse X-ray emission,
particularly that of the iron lines from neutral and highly ionized atoms, is a
superposition of many cataclysmic variables and coronally active binaries. In
this scenario, the flux should symmetrically distribute between the east and
west on the plane with respect to Sagittarius A$^*$ because the stellar mass
distribution determined by infrared observations is nearly symmetric. This
symmetry is confirmed for the highly ionized iron line as well as the continuum
emission. However, a clear excess of the neutral iron line in the near east of
the Galactic center compared to the near-west side is found. The flux
distribution of the excess emission well correlates with molecular column
density. The X-ray spectrum of the excess emission is described by a power-law
continuum plus a 6.4 keV line with the large equivalent width of $\sim1.3$ keV,
which is hardly explained by the low-energy electron bombardment scenario. The
longitudinal and latitudinal distribution of the excess emission disfavors the
X-ray irradiation, neither by Sagittarius A$^*$ nor by nearby X-ray binaries.
Then the low-energy proton bombardment is the most probable origin although the
high energy density $\sim 80$ eV cm$^{-3}$ in 0.1-1000 MeV is required and
there is no conventional proton source in the vicinity.
