  We calculate next-to-leading-order (NLO) corrections to the $B\to\pi$
transition form factors at leading twist in the $k_T$ factorization theorem.
Light partons off-shell by $k_T^2$ are considered in the quark diagrams, in the
effective diagrams for the $B$ meson wave function defined with the effective
heavy-quark field, and in the effective diagrams for the pion wave function. It
is explicitly demonstrated that the infrared logarithms $\ln k_T^2$ cancel
between the above sets of diagrams, as deriving the $k_T$-dependent NLO hard
kernel from their difference. The infrared finiteness of the hard kernel
confirms the application of the $k_T$ factorization theorem to $B$ meson
semileptonic decays. The NLO pion wave function is identical to those
constructed from the pion transition and electromagnetic form factors,
consistent with its universality. Choosing the renormalization and
factorization scales lower than the $B$ meson mass, the NLO corrections are
under control: they amount only up to 30% of the form factors at large recoil
of the pion, when varying models for the meson wave functions.
