  The chiral phase transition is studied in an extended Nambu--Jona-Lasinio
model with eight-quark interactions. Equations for scalar and vector quark
densities, derived in the mean field approximation, are nonlinear and mutually
coupled. The scalar-type nonlinear term hastens the restoration of chiral
symmetry, while the scalar-vector mixing term makes the transition sharper. The
scalar-type nonlinear term shifts the critical endpoint toward the values
predicted by lattice QCD simulations and the QCD-like theory.
