  The higher derivative field theories are notorious for the stability problems
both at classical and quantum level. Classical instability is connected with
unboundedness of the canonical energy, while the unbounded energy spectrum
leads to the quantum instability. For a wide class of higher derivative
theories, including the extended Chern-Simons, other bounded conserved
quantities which provide the stability can exist. The most general gauge
invariant extended Chern-Simons theory of arbitrary finite order $n$ admits $(n
- 1)$-parameter series of conserved energy-momentum tensors. If the
$00$-component of the most general representative of this series is bounded,
the theory is stable. The stability condition requires from the free extended
Chern-Simons theory to describe the unitary reducible representation of the
Poincar\'e group. The unstable theory corresponds to nonunitary representation.
