  Screening properties of the quark gluon plasma are studied from Polyakov-loop
correlation in lattice QCD simulations with two flavors of improved Wilson
quarks at temperatures $T/\Tpc \simeq 1$--4 where $\Tpc$ is the pseudocritical
temperature. Using the Euclidean-time reflection symmetry and the charge
conjugation symmetry, we introduce various types of Polyakov-loop correlation
functions and extract screening masses in magnetic and electric sectors. We
find that the temperature dependence of the screening masses are well described
by the weak coupling expansion. We also find that a ratio of the screening
masses in the electric sector to the magnetic sector shows qualitative
agreement with a prediction from the dimensionally-reduced effective field
theory and the N=4 supersymmetric Yang-Mills theory at $1.3 < T/\Tpc < 3$.
