  In a recent paper (Abe S 2013 Phys. Rev. E 88 022142), a variational
principle has been formulated for spatiotemporally-fractional Fokker-Planck
equations and applied to derivations of their approximate analytic solutions
based on the L\'evy Ansatz. Here, the problem of the constraint associated with
normalization condition on a probability distribution behind the principle is
discussed. It is shown that the action functional possesses a specific
transformation property in terms of an auxiliary field and the constraint turns
out to have already been imposed implicitly in terms of such a structure.
