  We calculate the two-pion wave function in the ground state of the I=2
$S$-wave system and find the interaction range between two pions, which allows
us to examine the validity of the necessary condition for the finite-volume
method for the scattering length proposed by L\"uscher. We work in the quenched
approximation employing a renormalization group improved gauge action for
gluons and an improved Wilson action for quarks at $1/a=1.207(12) {\rm GeV}$ on
$16^3 \times 80$, $20^3 \times 80$ and $24^3 \times 80$ lattices. We conclude
that the necessary condition is satisfied within the statistical errors for the
lattice sizes $L\ge 24$ ($3.92 {\rm fm}$) when the quark mass is in the range
that corresponds to $m_\pi^2 = 0.273-0.736 {\rm GeV}^2$. We obtain the
scattering length with a smaller statistical error from the wave function than
from the two-pion time correlator.
