The viscoelastic behavior of thermoresponsive poly(N-isopropylacrylamide) (PNIPA) microgel was investigated as a function of the preparation temperature T-prep using a 5 MHz quartz crystal microbalance (QCM). The viscoelastic behavior was determined on the basis of the resonant frequency F and resistance R of the QCM by means of impedance analysis of the QCM. The viscoelastic behavior was found to vary as a function of T-prep and/or the network and hydration structures of the gels. Large changes were induced in the viscoelasticity in response to thermal stimuli in the gels prepared below the lower critical solution temperature (LCST) of PNIPA. The changes were due to the homogeneous network structure of the gels with a large amount of hydrated water. Changes in the viscoelasticity of the gels prepared above the LCST were much smaller due to the porous network of the gels with a small amount of hydrated water. The use of QCM enabled the precise observation of network and hydration structures of the gels during the stimuli-responsive process from the viewpoint of the viscoelasticity.