The surface roughness of polymer gels was investigated by tapping mode atomic force microscopy (AFM). The spongelike domains on N-isopropylacrylamide (NIPA) gels were directly observed in water. It was found that the surface domain structure was strongly affected not only by the degree of the homogeneities of polymer networks but also by the bulk phase transition in response to the change in the external temperature. The domain size of homogeneous gels prepared at the ice temperature was found to be much smaller than that of the inhomogeneous gels prepared at a temperature above the cloud point of NIPA polymers. The surface structure was reproducibly observed at exactly the same position below and above the volume phase transition temperature, and its change was found to be reversible with temperature. The surface roughness due to the spongelike domains was discussed in terms of the autocorrelation function, the root-mean-square roughness, and the power spectral density, which were calculated from the AFM images.