We examined the formation of microcellular foam in a thermoplastic polyurethane elastomer (TPU) with supercritical carbon, dioxide. Measurements of the permeability and impregnation with carbon dioxide suggested that the impregnation of TPU with carbon dioxide was affected by the soft-segment chain length and soft-segment concentration in TPU and that carbon dioxide was mainly present in the soft-segment area. Thus, it seemed likely that the nucleation and growth of the nuclei in the microcells occurred in the soft-segment area. In addition, scanning electron microscopy of the microcells indicated that the higher the saturation pressure was, the smaller the mean cell diameter was; that is, the higher the cell number density was, the lower the foaming temperature was, the smaller the mean cell diameter was, and the lower the saturation pressure was, the more conspicuous this phenomenon was. These results showed that the saturation pressure and foaming temperature affected the cell structure.