The shrinking kinetics of poly(N-isopropylacrylamide) aqueous gels due to temperature jumps (T-jumps) across its volume phase transition temperature has been investigated as a function of the monomer concentration (C-NIPA), the destination temperature (Td), and the cross-linker concentration (C-BIS) The following facts were disclosed. (1) The shrinking rate is T-d dependent and is slowest for 40 less than or equal to T-d less than or equal to 45 degrees C. (2) By reducing C-NIPA to the lowest concentration for gel formation (i.e., C-NIPA approximate to 300 mM), an anomalous rapid shrinking is observed. (3) The shrinking rate becomes smaller with increasing C-BIS (4) Whenever a rapid shrinking takes place, characteristic changes in morphology, i.e., bubble formation, are observed on the surface of the gel. A mechanism of rapid shrinking of gels is proposed by taking account of bubble formation and phase separation.