We report pH-responsive structural changes in PEG-modified (PEGylated) nanogels, as determined by using small-angle X-ray scattering and dynamic light scattering. The size of the nanogels discontinuously increased at a lower pH than the pKa of the nanogels. This size increase was attributed to the swelling of the core part of the nanogel upon a change in pH. The swelling behavior was dependent on the cross-link density of the core. When the cross-link density was low, the core swelled greatly with preserving the polydispersity in size and maintained a constant shape; however, when the cross-link density was high, the core swelled minimally, and only the polydispersity increased. This difference in swelling behavior is discussed in terms of the inhomogeneous structural distribution of cross-links in the core.