  Phase transitions in the imaginary chemical potential region are studied by
the Polyakov loop extended Nambu-Jona-Lasinio (PNJL) model that possesses the
extended Z3 symmetry. The extended Z3 invariant quantities such as the
partition function, the chiral condensate and the modifed Polyakov loop have
the Roberge-Weiss (RW) periodicity. There appear four types of phase
transitions; deconfinement, chiral, Polykov-loop RW and chiral RW transitions.
The orders of the chiral and deconfinement transitions depend on the presence
or absence of current quark mass, but those of the Polykov-loop RW and chiral
RW transitions do not. The scalar-type eightquark interaction newly added in
the model makes the chiral transition line shift to the vicinity of the
deconfiment transition line.
