We succeeded in developing the highly porous Cellulose Nanofiber nonwoven fabrics sheet, CNF sheet, composed of cellulose nanofibers (average fiber diameter: ranging of 30- 400 nm). CNF sheet has multilayered structure constituted of network plane of nanofibers over at least 50 layer. Therefore, with increasing in weight of CNF sheet, the pore size becomes to decrease and have the narrower pore size distribution. CNF sheet is corresponding to a heat resistant material (usable up to 200 °C, showing no glass transition temperature, low coefficient of thermal expansion (CTE): below 10 ppm/K as the CTE value between room temperature and 200 °C), and has very small pore size (maximum pore size: ranging of 0.06-2 micro meters), furthermore, has very large surface area (specific surface area by B.E.T. method: 7-100 m²/g). These unique features of CNF sheet would lead to the applications in wide industrial field, such as functional filter, substrate material for a low CTE hybrid film and for a medical use.