Swelling behavior of nanocomposite hydrogels (NC gels) having organic polymer/inorganic clay network structures were systematically investigated, focusing on the role of exfoliated clay platelets with sodium counterions in the network and effects of various swelling conditions. NC gels in water exhibited characteristic swelling-deswelling behavior, i.e., initial large swelling, maximum swelling, and subsequent deswelling toward an equilibrium state, under conditions where the water was changed frequently. Effects of swelling conditions, such as the frequency of changing the water, amount of water per unit gram of gel, salt concentration, and pH of the swelling solvent, gel composition (e.g., kind of polymer, clay concentration (C(clay)), and polymer concentration (C(p))), swelling temperature, and gel size, were clarified. NC gels with different polymers were all found to exhibit swelling-deswelling behavior except at very low C(clay) and high C(p). Spontaneous deswelling of the gels was attributed to the combined effects of high swelling capability of the NC gel as a polyelectrolyte gel and continuous release of sodium ions from the network during swelling. Furthermore, the swelling-deswelling behavior could be reversed by reintroducing sodium ions into the network. These characteristic swelling behaviors of NC gels with polymer/clay networks are completely different from those of hydrogels with chemically cross-linked polymer network structures.