Effects of pressure on the phase separation and structure of weakly charged poly(N-isopropylacrylamide-co-acrylic acid) (PNIPA-AAc) solutions and gels with 7 wt % were investigated by small-angle neutron scattering (SANS). The phase diagrams were determined in the pressure-temperature (P-T) plane for the solution and gel with a light scattering method, which were parabolic functions, having maxima at (P = 38.1 MPa, T = 37.1 degreesC; for the solution) and (121.6 MPa, 51.6 degreesC; for the gel). SANS experiments were carried out according to the P-T phase diagrams. At the low temperature and pressure region, where the systems were in one phase, the scattering intensity functions, I(q)s, were well described with an Ornstein-Zernike function. However, a scattering maximum appeared in I(q)s at elevated temperatures and pressures. The regions where a scattering maximum was observed at T = 45 degreesC were P < 70 MPa and 200 MPa < P for the gel and P < 70 MPa for the solution. This scattering maximum is due to microphase separation as a result of antagonism between the electrostatic and hydrophobic interactions. In the case of the solution, an upturn in I(q) at the low q region was observed, indicating occurrence of macrophase separation in addition to microphase separation. For the polymer gels, on the other hand, such an upturn in I(q) was not observed as a result of pinning effect of cross-links. The phase behavior and critical phenomena of PNIPA-AAc gels and solutions will be discussed from the viewpoint of pressure effects on the hydrophobic interaction.