The scattered light intensity as a function of solvent pressure and the equilibrium swelling curve under the high pressure (900 atm) are measured for N-isopropylacrylamide gels. The concentration fluctuations in the gel diverges at a certain pressure which depends on temperature of the gel. The spinodal temperature is obtained as a function of solvent pressure. The divergence of concentration fluctuations induced by pressure is explained by the mean-field theory of phase separation in a binary mixture, which is reported by Tanaka et al., by taking account of the pressure dependence of the polymer solvent interaction parameter. The pressure dependence of the parameter is estimated from the pressure dependence of the swelling curve.