  We study scaling behavior of a chiral order parameter in the low density
region, performing a simulation of two-flavor QCD with improved Wilson quarks.
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. Applying the reweighting method and calculating derivatives of the chiral
order parameter with respect to the chemical potential, the scaling properties
of the chiral phase transition are discussed in the low density region. We
moreover calculate the curvature of the phase boundary of the chiral phase
transition in the temperature and chemical potential plane assuming the O(4)
scaling relation.
