  Within the general framework of SU(2) Nambu-Jona-Lasinio model beyond
mean-field approximation, the S-wave $\pi - \pi$ scattering lengths $a_0^0$ and
$a_0^2$ are calculated. The numerical results are in agreement with that
analyzed from recent experiment $\pi N \to \pi \pi N$ data by using
Chew-Low-Goebel technique, and the corresponding chiral condensate $<{\bar
q}q>$ is around $-(250 {\rm MeV})^3$, which is close to the ones obtained from
lattice and sum rules.
