We have studied kinetics of polymerization-induced phase separation of epoxy solution in oligoethylene glycol using light scattering and an optical microscope. It was found that special double droplet morphology was observed only in a very narrow range of epoxy fraction, which depended on the reaction temperature. In the narrow range the phase separation rate was very fast, and the bicontinuous structure which was typical in spinodal decomposition-type phase separation was observed only in the very early stage, suggesting that the solutions in the narrow fraction range entered directly the spinodal decomposition region without crossing the bimodal region. It is therefore considered that the temperature-dependent narrow fraction range corresponds to the trajectory of the critical point of the phase separation curve in the fraction-temperature diagram.