  We study the spontaneous Lorentz symmetry breaking in a field theoretical
model in (2+1)-dimension, inspired by string theory. This model is a gauge
theory of an anti-symmetric tensor field and a vector field (photon). The
Nambu-Goldstone (NG) boson for the spontaneous Lorentz symmetry breaking is
identified with the unphysical massless photon in the covariant quantization.
We also discuss an analogue of the equivalence theorem between the amplitudes
for emission or absorption of the physical massive anti-symmetric tensor field
and those of the unphysical massless photon. The low-energy effective action of
the NG-boson is also discussed.
