  We re-examine three-dimensional gauge theory with a Chern-Simons term in
which the Lorentz invariance is spontaneously broken by dynamical generation of
a magnetic field. A non-vanishing magnetic field leads, through the
Nambu-Goldstone theorem, to the decrease of zero-point energies of photons,
which accounts for a major part of the mechanism. The asymmetric spectral flow
plays an important role. The instability in pure Chern-Simons theory is also
noted.
