207It is shown that the stochastic quantization method of Parisi and Wu determines the vacuum structure of the spontaneously broken theories by the stochastic Schwinger-Dyson equation which, in the equilibrium limit, reduces to the ordinary Schwinger-Dyson equation and is solved in Pi-expansion of the contributing stochastic diagrams. The method explains the nonanalytic behavior, or the two-valuedness of order parameter at vanishing source. It incidentally avoids the problem of complexity and convexity of the effective potential in a totally different manner. The stochastic Schwinger· Dyson equation for the growing order parameter is explicitly solved in some simple cases including a )..if/ theory with both unbroken and spontaneously broken potentials, the Goldstone and the Nambu-Jona-Lasinio models in the lowest order approximation. Ward-Takahashi identities in the spontaneously broken theories are also derived using the Langevin equation without recourse to the path-integral nor the operator formalisms. § 1. IntroductionIn addition to the canonical and path-integral formalisms the stochastic quantization of Parisi and Wu 1 ) provides the third alternative of the second-quantization of field theory. It is based on the purely classical notions using the Langevin equation well-known in the theory of the Brownian motion. Thus field variables are considered to be the random ones subject to the Langevin equation with respect to an extra time coordinate (or the fifth-time) r, reproducing quantum Green's functions in the hypothetical thermodynamic limit r--'>CO. In euclidean formulation the nonperturbative proof of the equivalence of the method to the conventional quantization schemes is well-established.The Parisi-Wu formalism has been applied 2 ) to various problems in field theory including both non-gauge and gauge theories. In this method the Boltzmann factor e-s in the path-integral formula can be derived from the Langevin equation, but one need not work in the path-integral, thereby avoiding the precise definition of the measure. This is interesting because the path-integral bothers us with the problem of the measure. In this connection we recall gauge theories and anomalies. The purpose of this paper is to apply the stochastic quantization to reformulate the spontaneous breakdown of symmetry, which leads to the concept of the growing order parameter 3 ) in the spontaneously broken theories. It turns out that the Parisi-Wu method is capable of describing the spontaneous symmetry breaking without modifying the symmetric lagrangian, but a set of the stochastic diagrams in broken theories is generated by introducing a constant linear source. The result is formulated by an evolution equation for the growing order parameter, which becomes a quasi-linear, partial differential equation in the tree approximation. We solve the quasi-linear, partial differential equation by the method of the characteristic equations and find