Stress relaxation process of cross-linked sodium polyacrylate was investigated. When expression or expansion are stopped before a material reaches its equilibrium state, the stress of the material will decrease/increase as the material relaxes. The change of stress was measured after interruption of a constant-pressure or constant-rate expression/expansion operation of a packed spherical gel bed. It was found that the equilibrium stress of the material does not depend on the mode of expression/expansion (constant-pressure or constant-rate expression/expansion) but is uniquely determined by the average consolidation/expansion ratio when the expression/expansion is interrupted. This relaxation process was analyzed by use of the effective osmotic pressure, that is, the difference between the swelling pressure of the gel and the pressure applied to the gel. The reasonable agreement between calculated and empirical stress supports the validity of the analysis.