  We study non-equilibrium stationary states of a cavity system consisting of
many atoms interacting with a quantized cavity field mode, under a driving
field in a dissipative environment. We derive a quantum master equation which
is suitable for treating systems with a strong driving field and a strong
atom-photon interaction. We do this by making use of the fact that the
mean-field dynamics are exact in the thermodynamic limit thanks to a uniform
coupling between atoms and photons. We find ordered states with symmetry-broken
components of the photon field and atomic excitation driven by the external
field. The mechanism by which these ordered states arise is discussed from the
viewpoint of the quantum interference effect.
