The salt effect on the phase transition of N-isopropylacrylamide (NIPA) gel was studied. The swelling behavior of the NIPA gel strongly depends on the salt concentration and is well described as a function of the chemical potential difference of water molecules in solution from that at the transition. From the analysis of the OH stretching, Raman spectra in water and in various aqueous solutions in terms of collective proton motions reveals that the presence of salts tends to disrupt or distort the water molecules in hydrophobic hydration shell around the NIPA gel. This leads to inducing the growth of the cluster shell around the salts, which leads to gel collapse. The volume phase transitions due to the different types of perturbation (temperature, salt) are induced by the same mechanism, hydrophobic hydration and dehydration, and therefore can be described in a unified manner in terms of the chemical potential and the collective proton motions of water molecules.