Internal structures of translucent poly(N-isopropylacrylamide) gel were for the first time observed by laser scanning confocal microscopy (LSCM). The observed image reflected the mesoscopic internal structures in the gel due to the "fixed concentration fluctuations" of network polymers as described in the text. Three-dimensional internal structures in the gel constructed from a series of the cross section images taken by the LSCM revealed that the gel had a bicontinuous domain structure consisting of regions dense and sparse in polymer networks.