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.
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