  We argue that double logarithmic corrections $\alpha_s\ln^2 x$ need to be
resumed in perturbative QCD factorization theorem for exclusive $B$ meson
decays, when the end-point region with a momentum fraction $x\to 0$ is
important. These double logarithms, being of the collinear origin, are absorbed
into a quark jet function, which is defined by a matrix element of a quark
field attached by a Wilson line. The factorization of the jet function from the
decay $B\to\gamma l\bar\nu$ is proved to all orders. Threshold resummation for
the jet function leads to a universal, {\it i.e.}, process-independent, Sudakov
factor, whose qualitative behavior is analyzed and found to smear the end-point
singularities in heavy-to-light transition form factors.
