Using the hamiltonian formalism, we investigate the smooth bosonization method in which bosonization and fermionization are carried out through a specific gauge-fixing of an enlarged gauge invariant theory. The generator of the local gauge symmetry, which cannot be derived from the lagrangian of the enlarged theory, is obtained by making a canonical transformation. We also show that the massless Thirring model possesses a similar local gauge symmetry for a speific value of the coupling constant.