The swelling equilibria of N-substituted acrylamide gels, such as N-isopropylacrylamide, N-n-propylacrylamide, N-cyclopropylacrylamide, N-ethoxyethylacrylamide gels, were measured in water at various temperature. It was found that the gels showed thermoshrinking swelling behaviors and that the swelling behaviors were strongly dependent on the N-substitutes. The swelling equilibria of the N-isopropylacrylamide gel were measured in the presence of additives (salts and organic compounds). The additives did not affect the swelling ratio but depressed the transitin temperature. All these results were discussed in terms of hydrophobic interaction and additive effects on the hydration structure and it was confirmed that the hydrophobic interaction plays an important role in the thermoshrinking type phase transition. In addition, calculation results with a lattice model by Prange et al. were also presented.