Theoretical formulation for the thermodynamics of a nondilute polymer solution in a confined space is presented. The interactions between polymer chains were taken into account as an effective potential field in which a chain of the Gaussian conformation grows. We determined the potential so that, when applied to the bulk solution, the theory reproduces the osmotic pressure of the bulk solution calculated within renormalization group theory. The Green function for the polymer chains confined to a slit was calculated self-consistently, since the density profile obtained from the Green function determines the potential field within the slit. The chemical potential, the partition coefficient, the density profile, and the dimension of the polymer chain perpendicular to the slit walls were obtained as a function of the concentration for different slit widths. The results agree with those obtained in the computer simulations.