The pressure-induced volume phase transitions of hydrogels (poly (N-isopropylacrylamide) (pNIPA) and polyacrylamide) in mixed solvents for pressures up to 400 MPa were investigated. For an ionized pNIPA gel in methanol-water mixtures, the discontinuous volume phase transition, which occurred at a few tens MPa, was scarcely affected by methanol concentration. By contrast, the gradual shrinking, which occurred above 100 MPa, disappeared by adding a small amount of methanol. A polyacrylamide gel in acetone-water mixtures also underwent a volume phase transition at a phase transition pressure which depended greatly on acetone concentration, but did not the gradual shrinking. These pressure-induced volume phase transitions were interpreted by considering the pressure dependence of the interaction parameter between the solute (methanol or acetone) and water.