[en] For the 'anomalous' J/ψ suppression in Pb-Pb collision announced by NA50 Collaboration I propose new mechanism accessible to the novel behavior of the data. The point of new idea is to focus on in-medium effects on the masses of mesons containing charmed quarks. In QCD sum rule analysis, I find that the D-meson involving one charmed quark feels more attractive force from the surrounding nucleons than the charmonium composed of two charmed quarks. As a result, it will appear that as the nucleon density increases, DD-bar threshold in turn falls below higher-lying charmonium states (ψ', χ_c) which have the energy levels below the threshold in vacuum. Finally, also for the low-lying J/ψ state, a larger decay channel to the threshold could be opened up. Thus the mechanism can qualitatively explain some features of the anomalous J/ψ suppression without an advent of deconfinement phase. (author)