  We present an exploratory lattice study of $B\to\pi$ semileptonic decay form
factors using the nonrelativistic lattice QCD for heavy quark with Wilson light
quark on a $16^3 \times 32$ quenched lattice at $\beta=5.8$. The matrix
elements are calculated at eight values of heavy quark mass in a range of
$1.5-8$ GeV and with three values of light quark mass. The $1/m_B$ corrections
to the matrix elements are found to be fairly small except for the spatial
component proportional to the $B$ meson momentum. We find that the $q^2$
dependence of the form factor $f^+(q^2)$ near $q_{max}^2$ becomes much stronger
for larger heavy quark mass, which may suggest the increase of a pole
contribution. We perform a model independent fit of the form factors and study
whether the $q^2$ dependence is consistent with the pole contribution. Although
the soft pion theorem predicts $f^0(q_{max}^2)=f_B/f_{\pi}$ in the chiral
limit, we observe a significant violation of this relation.
