  We present a report on a calculation of scattering length for I=2 $S$-wave
two-pion system from two-pion wave function. Calculations are made with an
RG-improved action for gluons and improved Wilson action for quarks at
$a^{-1}=1.207(12) {\rm GeV}$ on $16^3 \times 80$, $20^3 \times 80$ and $24^3
\times 80$ lattices. We investigate the validity of necessary condition for
application of L\"uscher's formula through the wave function. We find that the
condition is satisfied for lattice volumes $L\ge 3.92 {\rm fm}$ for the quark
mass range $m_\pi^2 = 0.273-0.736 {\rm GeV}^2$. We also find that the
scattering length can be extracted with a smaller statistical error from the
wave function than with a time correlation function used in previous studies.
