  The free energy between a static quark and an antiquark is studied by using
the color-singlet Polyakov-line correlation at finite temperature in lattice
QCD with 2+1 flavors of improved Wilson quarks. From the simulations on $32^3
\times 12$, 10, 8, 6, 4 lattices in the high temperature phase, based on the
fixed scale approach, we find that, the heavy-quark free energies at short
distance converge to the heavy-quark potential evaluated from the Wilson loop
at zero temperature, in accordance with the expected insensitivity of short
distance physics to the temperature. At long distance, the heavy-quark free
energies approach to twice the single-quark free energies, implying that the
interaction between heavy quarks is screened.
  The Debye screening mass obtained from the long range behavior of the free
energy is compared with the results of thermal perturbation theory.
