Abstract: In consequence of the spontaneous symmetry breaking, non-zero energy Goldstone, modes with dissipation are excited in a non-equilibrium stationary state with space-time structure. In this paper, as a specific example, the Ward-Takahashi identities formulated in the close time path Green's function method is applied to the saturation state of a single mode laser. A generalized Goldstone theorem in a weak inho-mogeneous dissipative system is established and the physical interpretation of the Goldstone mode is discussed. As a result of the Goldstone theorem, the pole in the Green's function of the laser light splits into two with equal weights, each corresponding to a quanta with the same frequency but different dissipation. Together with the order parameter (the average value of the vector potential), these two kinds of quanta (one of which is the Goldstone mode) give a complete description of the order-disorder transition of the phase symmetry in the saturation state of the laser. A detailed discussion on the restoration of the spontaneously broken symmtry of the phase is given.