  We calculate next-to-leading-order (NLO) corrections to exclusive processes
in $k_T$ factorization theorem, taking $\pi\gamma^*\to\gamma$ as an example.
Partons off-shell by $k_T^2$ are considered in both the quark diagrams from
full QCD and the effective diagrams for the pion wave function. The gauge
dependences in the above two sets of diagrams cancel, when deriving the
$k_T$-dependent hard kernel as their difference. The gauge invariance of the
hard kernel is then proven to all orders by induction. The light-cone
singularities in the $k_T$-dependent pion wave function are regularized by
rotating the Wilson lines away from the light cone. This regularization
introduces a factorization-scheme dependence into the hard kernel, which can be
minimized in the standard way. Both the large double logarithms $\ln^2k_T$ and
$\ln^2 x$, $x$ being a parton momentum fraction, arise from the loop correction
to the virtual photon vertex, the former being absorbed into the pion wave
function and organized by the $k_T$ resummation, and the latter absorbed into a
jet function and organized by the threshold resummation. The NLO corrections
are found to be only few-percent for $\pi\gamma^*\to\gamma$, if setting the
factorization scale to the momentum transfer from the virtual photon.
