  Thermodynamics of two-flavor QCD at finite temperature and density is studied
on a $16^3 \times 4$ lattice, using a renormalization group improved gauge
action and the clover improved Wilson quark action. In the simulations along
lines of constant $m_{\rm PS}/m_{\rm V}$, we calculate the Taylor expansion
coefficients of the heavy-quark free energy with respect to the quark chemical
potential ($\mu_q$) up to the second order. By comparing the expansion
coefficients of the free energies between quark($Q$)and antiquark($\bar{Q}$),
and between $Q$ and $Q$, we find a characteristic difference at finite $\mu_q$
due to the first order coefficient of the Taylor expansion. We also calculate
the quark number and isospin susceptibilities, and find that the second order
coefficient of the quark number susceptibility shows enhancement around the
pseudo-critical temperature.
