  We study the equation of state in two-flavor QCD at finite temperature and
density. Simulations are made with the RG-improved gluon action and the
clover-improved Wilson quark action. Along the lines of constant physics for
$m_{\rm PS}/m_{\rm V} = 0.65$ and 0.80, we compute the derivatives of the quark
determinant with respect to the quark chemical potential $\mu_q$ up to the
fourth order at $\mu_q=0$. We adopt several improvement techniques in the
evaluation. We study thermodynamic quantities and quark number susceptibilities
at finite $\mu_q$ using these derivatives. We find enhancement of the quark
number susceptibility at finite $\mu_q$, in accordance with previous
observations using staggered-type quarks. This suggests the existence of a
nearby critical point.
