  Supernova remnants (SNRs) have been prime candidates for Galactic cosmic-ray
accelerators. When low-energy cosmic-ray protons (LECRp) collide with
interstellar gas, they ionize neutral iron atoms and emit the neutral iron line
(Fe I K$\alpha$) at 6.40keV. We search for the iron K-shell line in seven SNRs
from the Suzaku archive data of the Galactic plane in the $6^{\circ} \lesssim l
\lesssim 40^{\circ}, |b| < 1^{\circ}$ region. All these SNRs interact with
molecular clouds. We discover Fe I K$\alpha$ line emissions from five SNRs
(W28, Kes 67, Kes 69, Kes 78, and W44). The spectra and morphologies suggest
that the Fe I K$\alpha$ line is produced by interactions between LECRp and the
adjacent cold gas. The proton energy density is estimated to be $\gtrsim$
10-100 eV cm$^{-3}$, which is more than 10 times higher than that in the
ambient interstellar medium.
