  We study the spin-gap phase in the one-dimensional t-J model, assuming that
it is caused by the backward scattering process. Based on the renormalization
group analysis and symmetry, we can determine the transition point between the
Tomonaga-Luttinger liquid and the spin-gap phases, by the level crossing of the
singlet and the triplet excitations. In contrast to the previous works, the
obtained spin-gap region is unexpectedly large.
  We also check that the universality class of the transition belongs to the
$k=1$ SU(2) Wess-Zumino-Witten model.
