Simulation of polymerization-induced phase separation of a polymer gel from a multifunctional monomer was carried out using the phase field method. The mobility was assumed to depend negatively and sigmoidally on the volume fraction of the polymer, considering the steric rigidity of the polymeric species of the multifunctional monomer. The results showed that the volume ratio of the polymer phase and solvent phase was well responsive to the volume fraction of the polymer. The whole of the system was composed of the polymer phase, and no solvent phase was formed even when the volume fraction of the polymer was less than 0.6. It was also revealed that the solvent phase formed a bimodal morphology, in which the smaller solvent phases were dispersed in the polymer phases, while the larger solvent phases had a continuous or dispersed morphology. Inversion of the gradient of chemical potential near the interface between the larger solvent phase and the polymer phase was proposed as a mechanism for the formation of the bimodal morphology. The feature of the phase structure of the macroporous silica prepared via polymerization-induced phase separation qualitatively agreed with the simulation results.