  We present a preliminary report on a calculation of scattering length for I=2
$S$-wave two-pion system directly from the two-pion wave function. Results are
compared with those calculated from the time dependence of two-pion four-point
functions. Calculations are made with an RG-improved action for gluons and
improved Wilson action for quarks at $a^{-1}=1.207(12)$ GeV on $20^3 \times 48$
and $24^3 \times 48$ lattices.
