Using the generalized method of self-consistent field developed by one of the authors[4], we discuss various correlation phenomena of many-body systems with a unified viewpoint. The general method of treatment is proposed and compared with the method of perturbation. From the approximation with two-body correlation we obtain, as particular applications, the Bruckner theory of nuclear matter and the Bohm-Pines theory of the collective oscillation of electron,gases. Finally the superconductivity and zero-point oscillations in non-ideal gases are discussed by solving, the bound state problem with the present method and the Gell-mann Low procedure. It is concluded that the method is very effective in treating correlation phenomena.