A series of inhomogeneities were introduced to poly(methyl methacrylate) (PMMA) gels by carrying out the polymerization of PMMA network in the presence of various amounts of polystyrenes (PS) (M-w = 400 000). The pulsed-field-gradient stimulated-echo H-1 NMR measurements were performed for the PMMA gel samples with variable diffusion time A. which enabled us to investigate the PMMA gel network structure on the basis of the diffusional behavior of the large probe molecule (PS) and a small probe molecule (unreacted monomer) in PMMA gels. The diffusion behavior of the large PS probe Molecule proved to significantly dependent on the amount of PS that had been added in the gelation batch. The analysis of PGSE H-1 NMR measurements strongly suggested that the PS diffusion consists of two diffusion components, while it changes to a single mode with increasing the PS concentration. These results were ascribed to the Course of the phase separation resulting from development of "open" network structure that was induced by the PS probe molecule in the gelation batch.