  Critical properties in the dynamical charge correlation function for the
one-dimensional Mott insulator are studied. By properly taking into account
{\it the final-state interaction} between the charge and spin degrees of
freedom, we find that the edge singularity in the charge correlation function
is governed by massless spinon excitations, although it is naively expected
that spinons do not directly contribute to the charge excitation over the
Hubbard gap. We obtain the momentum-dependent anomalous critical exponent by
applying the finite-size scaling analysis to the Bethe ansatz solution of the
half-filled Hubbard model.
