A partial differential equation in time and space variables for the BCS order parameter for a non-equilibrium system is derived within the framework of nonperturbative canonical quantum many-body theory. The derivation is rigorous and no approximation is employed. The ensemble average used to define the order parameter is characterized by an arbitrary density matrix that satisfies the quantum Liouville equation. §1. IntroductionSince Ginzburg and Landau proposed an extremely successful phenomenological model of superconductivity, 1) formulating a microscopic derivation of that model has been one of the central problems in the study of superconductivity. 2) Two years after the proposal of the BCS theory, 3) Gor'kov refined it by introducing "anomalous" Matsubara Green functions 4) and derived the Ginzburg-Landau model with various approximations for temperatures near T c . 5) An extension of the Gor'kov theory to the time-dependent case was carried out by Abrahams and Tsuneto, 6) who used the Martin-Schwinger theory of Matsubara Green functions 7) and the Kadanoff-Baym formulation of non-equilibrium quantum many-body problems. 8) They carried out a Taylor expansion of the integral kernel in the integral form of the Gor'kov equation in momentum-frequency space and obtained a diffusion-type equation in the vicinity of the critical temperature and a wave-like equation at zero temperature for the superconducting order parameter.Recently, Stoof 9) reformulated Abrahams-Tsuneto theory using the Keldysh theory 10) in the Grassmann number 11) path-integral form and the Hubbard-Stratonovich transformation 12)-14) to introduce the pair fields and derived an effective Lagrangian for the phase of the order parameter. Stoof's effective Lagrangian yields a wave equation. Aitchison et al. 15) criticized Stoof's work on the ground that it violates Galilean invariance and derived an effective Lagarangian for the phase of the order parameter, which is manifestly Galilean invariant and very different from Stoof's result. From the effective Lagrangian, they constructed a density and a current in terms of the phase variable. Then, they identified the square root of the density as the modulus of the complex order parameter. They showed that a nonlinear Schrödinger Lagrangian for the complex order parameter actually yields the assumed forms of the density and current. Their result strongly suggests that the equation describing the behavior of the superconducting order parameter is a nonlinear Schrödinger equation. Their work was further developed for application to finite temperatures. 16), 17) De Palo et al. extended their work by introducing the density, which was introduced