We evaluate scattering amplitudes at on-shell and half off-shell for $I=2$ S-wave two-pion system using the Bethe-Salpeter wave function inside the interaction range in the quenched QCD. The scattering length and effective range are extracted from these scattering amplitudes. Quark mass dependence of them is investigated with the pion mass ranged in $0.52$--$0.86$~GeV. We examine consistency between a result by the conventional finite volume method and our estimate, as well as the phenomenological value.
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