  The average phase factor of the QCD determinant is evaluated at finite quark
chemical potential ({\mu}_q) with the two-flavor version of the Polyakov-loop
extended Nambu-Jona-Lasinio (PNJL) model with the scalar-type eight-quark
interaction. For {\mu}_q larger than half the pion mass at vacuum m_{\pi}, the
average phase factor is finite only when the Polyakov loop is larger than 0.5,
indicating that lattice QCD is feasible only in the deconfinement phase. A
critical endpoint (CEP) lies in the region of the zero average phase factor.
The scalar-type eight-quark interaction makes it shorter a relative distance of
the CEP to the boundary of the region. For {\mu}_q < m_{\pi}/2, the PNJL model
with dynamical mesonic fluctuations can reproduce lattice QCD data below the
critical temperature.
