  We evaluate $I=2$ two-pion scattering length through the scattering amplitude
obtained by the Bethe-Salpeter wave function inside the interaction range. The
scattering length is computed with $m_\pi = 0.52-0.86$ GeV in the quenched
lattice QCD. Furthermore, the half-off-shell amplitude is calculated, from
which the effective range is extracted. Our results are compared with those by
the conventional finite size method and by chiral perturbation theory to
confirm consistency.
