Based on the Schwinger-Dyson (SD) equation, the fermion mass generatio
n is studied further in the D(2 < D < 4)-dimensional Thirring model as
a gauge theory previously proposed. Using a certain approximation on
the kernel, we analytically obtain explicit form of the dynamical mass
of the fermion and the critical line in (N, 1/g) space, where N is th
e number of fermions and g is the dimensionless vector-type four-fermi
on coupling constant. This analytical result is confirmed by the numer
ical solution for the SD equation with the exact form of the kernel in
(2+1) dimensions.