  A variational principle is developed for fractional kinetics based on the
auxiliary-field formalism. It is applied to the Fokker-Planck equation with
spatio-temporal fractionality, and a variational solution is obtained with the
help of the L\'evy Ansatz. It is shown how the whole range from subdiffusion to
superdiffusion is realized by the variational solution, as a competing effect
between the long waiting time and the long jump. The motion of the center of
the probability distribution is also analyzed in the case of a periodic drift.
