I=2 $\pi$-$\pi$ scattering length with dynamical overlap fermion

We report on a lattice QCD calculation of the I=2 $\pi \pi$ scattering length using the overlap fermion formulation for both sea and valence quarks. We investigate the consistency of the lattice data with the prediction of the next-to-next-to-leading order chiral perturbation theory after correcting finite volume effects. The calculation is performed on gauge ensembles of two-flavor QCD generated by the JLQCD collaboration on a ${16}^3\times 32$ lattice at a lattice spacing $\sim$ 0.12 fm.