Diffusional behaviors of probe polystyrenes (PSs) with M-w = 4K, 19K, 29K, and 400K and chloroform as solvent were observed in the same PMMA gel matrix with the diffusing time varied (4-500 ms) by diffusing-time-dependent H-1 NMR in order to elucidate how the same gel network structure is differently detected by the probes with different sizes. The diffusion of the solvent and the smallest PS with M, = 4K proved to be of a single mode, which is safely ascribed to the sufficiently small size of the probes compared with the network mesh (similar to 1.5 nm). The other PS molecules that should be larger than or comparable to the mesh size showed a multimode diffusional behavior, which was analyzed as a dual mode diffusion composed of a fast component and a slow one. Thus, those larger PS molecules were capable to "probe" the inhomogeneity for the mesh size distribution of the network. On the other hand, the so-called restricted diffusion or diffusing-time dependence for the diffusion coefficient D was observed only for the largest probe PS with Mw = 400K, and the fast diffusion component and the slow diffusion component with the diffusing time were recognized. This means that the PS chains were entrapped within a dense or an open structure during the measurement time, respectively. On the basis of these diffusion behaviors of probe molecules, the mesh and region sizes were estimated; for the open network structure the region size was scaled to be on the order of micrometers at least, while the mesh size of the dense region was to be larger than 1.5 nm and must be considerably smaller than 30 nm.