Fine porous structure with the size of several ten nm to several μm can be obtained by foaming of polymers using supercritical fluids such as supercritical carbon dioxide. Owing to the characteristic properties originated from the fine porous structure, porous materials obtained by supercritical foaming are expected to be applied to structural materials, medical materials, insulation materials, and electronic materials. Control of porous structure of rubbers using supercritical fluids is also briefly described.