The Sullivan model is adopted for the interaction parameters of fluid-fluid and wall-fluid potentials in binary mixtures. The capillary condensation in 'two-coexisting-phases' and 'three-coexisting-phases' open systems confined between two parallel walls is studied analytically by a dynamical approach. The result shows that not only a first-order but also a continuous transition is possible. The two systems studied in the present paper have no apparent difference in their wetting behaviours, and their phase diagram constructed in terms of the two components of the wall-fluid potential is a segment of an ellipse.