  We study scaling behavior of a chiral order parameter performing a simulation
of two-flavor QCD with improved Wilson quarks. It has been shown that the
scaling behavior of the chiral order parameter defined by a Ward-Takahashi
identity agrees with the scaling function of the three-dimensional O(4) spin
model at zero chemical potential. We extend the scaling study to finite density
QCD. Calculating derivatives of the chiral order parameter with respect to the
chemical potential in two-flavor QCD, the scaling property of chiral phase
transition is discussed in the low density region. We moreover calculate the
curvature of the phase boundary of the chirl phase transition in the
temperature and chemical potential plane assuming the O(4) scaling relation.
