Equilibrium swelling curves of ionized polyacrylamide gels immersed in acetone-water mixtures were measured as a function of pressure up to pressures of 300 MPa. The gels, which shrank at atmospheric pressure, underwent an abrupt volume change (pressure-induced volume phase transition) from a shrunken state to a swollen state at the transition pressure. The transition pressure increased with an increase of acetone concentration. The pressure-induced volume phase transition can be interpreted by taking account of the free-energy change Delta V . P between swollen (hydrated) and shrunken (dehydrated) states. The Delta V represents the difference between the molar volume of water structured around hydrophilic groups of polyacrylamide chains and that of free water in the bulk mixtures. The estimated value of Delta V is -3.3 mL/mol, which qualitatively agrees with that obtained from the experiments of denaturation of proteins. The pressure-induced volume phase transition is generally expected in many hydrogels.