  Effects of the vector-type four-quark interaction on QCD phase structure are
investigated in the imaginary chemical potential region, by using the
Polyakov-loop extended Nambu-Jona-Lasinio (PNJL) model with the extended Z3
symmetry. In the course to this end, we clarify analytically the Roberge-Weiss
periodicity and symmetry properties of various quantities under the existence
of a vector-type four-quark interaction. In the imaginary chemical potential
region, the chiral condensate and the quark number density are sensitive to the
strength of the interaction. Based on this result, we propose a possibility to
determine the strength of the vector-type interaction, which largely affects
QCD phase structure in the real chemical potential region, by comparing the
results of lattice simulations and effective model calculations in the
imaginary chemical potential region.
