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 PS /m V = 0.65 and 0.80, we compute the derivatives of the quark determinant with respect to the quark chemical potential µ q up to the fourth order at µ q = 0. We adopt several improvement techniques in the evaluation. We study thermodynamic quantities and quark number

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... ceptibilities at finite µ q using these derivatives. We find enhancement of the quark number susceptibility at finite µ q , in accordance with previous observations using staggered-type quarks. This suggests the existence of a nearby critical point.