  We give another reformulation of the Thirring model (with four-fermion
interaction of the current-current type) as a gauge theory and identify it with
a gauge-fixed version of the corresponding gauge theory according to the
Batalin-Fradkin formalism. Based on this formalism, we study the chiral
symmetry breaking of the $D$-dimensional Thirring model ($2<D<4$) with $N$
flavors of 4-component fermions. By constructing the gauge covariant effective
potential for the chiral order parameter, up to the leading order of $1/N$
expansion, we show the existence of the second order chiral phase transition
and obtain explicitly the critical number of flavors $N_c$ (resp. critical
four-fermion coupling $G_c$) as a function of the four-fermion coupling $G$
(resp. $N$), below (resp. above) which the chiral symmetry is spontaneously
broken.
