Invariant mass spectra of low-mass vector mesons in the e⁺e^- channel are measured in 12 GeV p + C and p + Cu reactions [Formula: see text] at the KEK Proton Synchrotron. Significant excess is observed at the low-mass side of ω meson peak over the known hadronic sources both in the C and Cu target data. Also below the ϕ meson peak, an excess is found in the Cu target data for slowly moving mesons whose βγ is less than 1.25. They can be interpreted as signatures of the spectral modification of vector mesons in cold nuclear matter.