  We evaluate the mass shift of isospin-averaged D-meson in the nuclear medium.
Borel-transformed QCD sum rules are used to describe an interaction between the
D-meson and a nucleon by taking into account all the lowest dimension-4
operators in the operator product expansion (OPE). We find at normal matter
density the D-meson mass shift is about 10 times (\sim 50 MeV) larger than that
of J/\psi. This originates from the fact that the dominant contribution in the
OPE for the D-meson is the nucleon matrix element of m_c\bar{q}q, where m_c is
the charm-quark mass and q denotes light quarks. We also discuss that the mass
shift of the D-meson in nuclear matter may cause the level crossings of the
charmonium states and the D\bar{D} threshold. This suggests an additional
mechanism of the J/\psi suppression in high energy heavy-ion collisions.
