Simulation of polymerization-induced phase separation of a polymer gel from a multifunctional monomer was carried out using the phase field method. To describe the driving force for phase separation, the interfacial energy between the polymeric phase and the aqueous phase was assumed to depend positively on the degree of polymerization, while the thickness of the interface at equilibrium and the mobility were assumed to depend negatively on it. The mobility was also assumed to depend negatively on the volume fraction of the polymer. A sigmoidal function was introduced to describe the nonlinear dependence of the mobility on the volume fraction. The wavelength of phase separation became finer as the polymerization rate increased when the rate of the reaction had a time scale similar to that of diffusion. When a nonreactive polymer coexists in the phase separation, the wavelength of phase separation shrinks because of suppression of diffusion caused by the coexisting polymer. These results qualitatively agreed with the experimental results for macroporous silica prepared via polymerization-induced phase separation.