  We extend our previous study of the QCD phase structure in the heavy quark
region to non-zero chemical potentials. To identify the critical point where
the first order deconfining transition terminates, we study an effective
potential defined by the probability distribution function of the plaquette and
the Polyakov loop. The reweighting technique is shown to be powerful in
evaluating the effective potential in a wide range of the plaquette and
Polyakov loop expectation values. We adopt the cumulant expansion to overcome
the sign problem in the calculation of complex phase of the quark determinant.
We find that the method provides us with an intuitive and powerful way to study
the phase structure. We estimate the location of the critical point at finite
chemical potential in the heavy quark region.
