  We study the critical behavior of the D (2<D<4) dimensional Gross-Neveu model
with a Thirring interaction, where a vector-vector type four-fermi interaction
is on equal terms with a scalar-scalar type one. By using inversion method up
to the next-to-leading order of 1/N expansion, we construct a gauge invariant
effective potential. We show the existence of the chiral order phase
transition, and determine explicitly the critical surface. It is observed that
the critical behavior is mainly controlled by the Gross-Neveu coupling g. The
critical surface can be divided into two parts by the surface g=1 which is the
critical coupling in the Gross-Neveu model at the 1/N next-to-leading order,
and the form of the critical surface is drastically change at g=1. Comparison
with the Schwinger-Dyson(SD) equation is also discussed. Our result is almost
the same as that derived in the SD equation. Especially, in the case of pure
Gross-Neveu model, we succeed in deriving exactly the same critical line as the
one derived in the SD equation.
