The shrinking mechanism of comb-type grafted poly(N-isopropylacrylamide) gel was investigated by fluorescence spectroscopy and small-angle X-ray Scattering (SAXS). The SAXS reveals that the microdomain structure with characteristic dimension of 460A is developed in the comb-type grafted poly(N-isopropylacrylamide) gel during the shrinking process. Fluorescence spectroscopy together with SAXS observation suggests that the freely mobile characteristics of the grafted chains are expected to show the rapid dehydration to make tightly packed globules with temperature, followed by the subsequent hydrophobic intermolecular aggregation of the dehydrated graft chains. The dehydrated grafted chains created the hydrophobic cores, which enhance the hydrophobic aggregation of the networks. These aggregations of the NIPA chains contribute to an increase in void volume, which allow the gel having a pathway of water molecules by the phase separation.