An attempt to analyze the pore structure of the cuprammonium regenerated cellulose hollow fiber (BMM hollow fiber) in order to clear up its filtration mechanism was made. The electron microscopy was employed to get the concrete images of the structure. The cellulose particles of rod-like shape with circular cross section having mean diameter of about 50 nm were its constructing units. The pores were classified into two types, i.e., the pore with the average diameter of about 50 nm and another with the diameter of several hundreds to several thousands nm. The former was estimated to be the capillary formed among neighboring cellulose particles and the latter to be the void formed as a vacant space which was originated by the phase separation as polymer lean phase. The frequency distribution curve of the void size showed several peaks indicating the occurrence of the boundary breakage between voids originated by the elongation of the fiber in the spinning process. The performances of BMM depends mainly on the existence of capillaries, then BMM with higher ability may be obtained by means of the spinning method which can decrease the occurrence of structure breakage due to the elongation during spinning.