Rejection characteristics of macromolecular gel layer for low-molecular weight solutes were studied by ultrafiltration using o¥ albumin and polyvinyl alcohol, which formed a gel layer, and vitamin B₁₂, raffinose and glucose, which were rejected by the gel layer. The bilayer membrane model and the Jagur-Grodzinski and Kedem equations were found effective in analyzing the rejection ability of the gel layer, which was defined by two parameters, σ_g and P_g. These parameters could be determined by the curve-fitting method. Furthermore, σ_g and P_g were analyzed by use of the steric hindrance-pore model, and the structure of the gel layer was estimated. It became clear from the experimental results that compaction of the gel layer by mechanical pressure occurred and affected the rejection ability. This effect can be calculated by taking into the pressure dependencies of thickness and pore radius of gel layer.