  Free energies between static quarks and Debye screening masses in the
quark-gluon plasma are studied on the basis of Polyakov-line correlations in
lattice simulations of 2+1 flavors QCD with the renormalization-group improved
gluon action and the $O(a)$-improved Wilson quark action. We perform
simulations at $m_{\rm PS}/m_{\rm V} = 0.63$ (0.74) for light (strange) flavors
with lattice sizes of $32^3 \times N_t$ with $N_t=4$--12. We adopt the
fixed-scale approach, where temperature can be varied without changing the
spatial volume and renormalization factor. We find that, at short distance, the
free energies of static quarks in color-singlet channel converge to the
static-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 free energies of static quarks approach to
twice the single-quark free energies, implying that the interaction between
static quarks is fully screened. The screening properties can be well described
by the screened Coulomb form with appropriate Casimir factor at high
temperature. We also discuss a limitation of the fixed-scale approach at high
temperature.
