  We determine the location of the critical point where the first-order
deconfining transition in the heavy-quark region turns into a crossover in
finite-temperature and density lattice QCD with 2+1 flavors of Wilson quarks.
Combining a hopping parameter expansion (HPE) of the quark determinant with a
reweighting method, we evaluate the chemical potential dependence of the
critical point. By systematically calculating the coefficients of the hopping
parameter expansion up to a high order of HPE at finite chemical potential, we
find that the higher order terms are strongly correlated with the Polyakov
loop, which is the leading-order term, on each configuration. Moreover, their
complex phases themselves, which are important at finite density, are also
found to be strongly correlated with the complex phase of the Polyakov loop.
Using this property, we develop a method for estimating the critical point
incorporating high-order terms from calculations with only low-order terms. We
report that the first-order phase transition region in the heavy-quark region
becomes narrower exponentially with increasing the chemical potential. Since
the hopping parameter of the critical point decreases exponentially as the
density increases, the sign problem does not become serious even when the
density increases, and critical points can be evaluated up to high densities.
