  Meson properties at finite temperature and density are studied in lattice QCD
simulations with two-flavor Wilson fermions. For this purpose, we investigate
screening masses of mesons in pseudo-scalar (PS) and vector (V) channels. The
simulations are performed on $16^3\times 4$ lattice along the lines of constant
physics at $m_{\rm PS}/m_{\rm V}|_{T=0}=0.65$ and 0.80, where $m_{\rm
PS}/m_{\rm V}|_{T=0}$ is a ratio of meson masses in PS and V channels at $T=0$.
A temperature range is $T/T_{\rm pc}=(0.8 - 4.0)$, where $T_{\rm pc}$ is the
pseudo-critical temperature. We find that the temperature dependence of the
screening masses normalized by temperature, $M_0/T$, shows notable structure
around $T_{\rm pc}$, and approach $2\pi$ at high temperature in both channels,
which is consistent with twice the thermal mass of a free quark in high
temperature limit. The screening masses at low density are also investigated by
using the Taylor expansion method with respect to the quark chemical potential.
We find that the expansion coefficients in the leading order become positive in
the temperature range, and thermal and density effect on the meson
screening-masses becomes apparent in the quark-gluon plasma phase. The meson
screening-masses are also compared with the gluon (Debye) screening masses at
finite temperature and density.
