Recently extensive investigations have been directed toward the critical phenomena occurring in the liquid column of binary mixtures in a manner similar to the case of a pure component. This thesis presents a self-consistent analytic theory concerning these phenomena for "simple" binary systems. In particular, this thesis discusses, (1) a detailed derivation of the equation of state concerning the gravity-induced inhomogeneity (2) power law dependence of the coexistence curve (3) geometrical realization of the coexistence curve in a miscibility gap (4) thermodynamic stabilities in a binary system. In addition, the possibility of applying the present theory to thermodynamic measurements and the analogy between unitary and binary critical equilibria are discussed.