  We investigate the canonical structure of the (2+1)-dimensional non-linear
$\sigma$ model in a $polynomial$ formulation. A current density defined in the
non-linear $\sigma$ model is a vector field which satisfies a $formal$ flatness
(or pure gauge) condition. It is the polynomial formulation in which the vector
field is regarded as a dynamical variable on which the flatness condition is
imposed as a constraint condition by introducing a Lagrange multiplier field.
The model so formulated has gauge symmetry under a transformation of the
Lagrange multiplier field. We construct the generalized Hamiltonian formalism
of the model explicitly by using the Dirac method for constrained systems. We
derive three types of the pre-gauge-fixing Hamiltonian systems: In the first
system, the current algebra is realized as the fundamental Dirac Brackets. The
second one manifests the similar canonical structure as the Chern-Simons or BF
theories. In the last one, there appears an interesting interaction as the
dynamical variables are coupled to their conjugate momenta via the covariant
derivative.
