  We study the curvature of the chiral transition/crossover line between the
low-temperature hadronic phase and the high-temperature quark-gluon-plasma
phase at low densities, performing simulations of two-flavor QCD with improved
Wilson quarks. After confirming that the chiral order parameter defined by a
Ward-Takahashi identity is consistent with the scaling of the O(4) universality
class at zero chemical potential, we extend the scaling analysis to finite
chemical potential to determine the curvature of the chiral
transition/crossover line at low densities assuming the O(4) universality. To
convert the curvature in lattice units to that of the $T_c(\mu_B)$ in physical
units, we evaluate the lattice scale applying a gradient flow method. We find
$\kappa=0.0006(1)$ in the chiral limit, which is much smaller than that
obtained in (2+1)-flavor QCD with improved staggered quarks.
