The method of correlation function is, without complete justification, extended to superconductors in the presence of a magnetic field. Applying this method, we derive the linearized and generalized GINZBURG—LANDAU equation and DE GENNES’ diffusion approximation in simple way. The results agree with those obtained previously by DE GENNES, GORKOV, MAKI, TEWORDT, and others. In special cases (no magnetic field, pure superconductor or isotropic scattering) they can also be derived from WERTHAMER’S kernel. In connection with this kernel, we discuss the limits of validity of both the linearized and generalized GINZBURG—LANDAU equation and of the diffusion approximation.