Microscopic phase separation and long-time relaxation processes have been studied for the three-component polymer blend solutions which are composed of polyethersulfone (PES), polysulfone (PSF), and N-methyl-2-pyrrolidone (NMP) as a solvent for both PES and PSF. The ternary systems of total polymer concentration of 30 wt % change from a clear state to a uniformly turbid state with temperature change. That is, the system has a cloud temperature as a lower critical solution temperature (LCST). The most characteristic behavior of the 30% mixed systems is that the uniformly turbid state is very stable and does not proceed to the macroscopic phase separation for a long time, i.e., more than several months. The mixed systems of PES/PSF = 3/7 and 5/5 show a long-time relaxation process due to the microscopic phase separation. On the other hand, the system of PES/PSF = 7/3 does not clearly show the long-time relaxation process in spite of the fact that the microscopic phase separation pattern is very similar to that of the system of 3/7. The long-time relaxation behavior is analyzed using the emulsion model for both the systems, and it revealed that the interfacial tension of the mixed systems is extremely small in comparison with the various blended polymer melts and also that the rheological measurements are very useful to estimate the interfacial tension and the characteristic time for the geometric deformation of the dispersing emulsion particles.