Inhomogeneity of network size of polymer gels such as polystyrene gel and cross-linked ethoxylate acrylate gel with dimethylformamide as solvent has been studied by using time-dependent diffusion NMR. From the experimental results on the diffusion coefficients of the probe amino acid, tertbutyloxylcarbonyl-(L)-phenylalanine, in the gels, it is found that, in the short diffusion time range, the amino acid in the gels has two components in diffusion as influenced by the distribution of network size, but in the long diffusion time range the amino acid has a single component in diffusion because the diffusion coefficients which come from the distribution of network size are averaged out. Then, inhomogeneity of the gels is elucidated.