A hemoconcentrator is installed as a part of cardiopulmonary bypass to concentrate the blood by removing excess water and unnecessary electrolytes from the blood diluted with myocardial protection fluid. The hemoconcentrator must remove water from diluted blood efficiently and quickly and remove proinflammatory cytokines and other unwanted molecules, without losing useful proteins such as albumin. Especially, the pore diameter and diameter distribution of the innermost surface greatly affect the pure water permeability and sieving coefficient of the solutes. In this study, the pore structure of the inner surface of the membrane was observed, and pore measurement of hollow fiber hemoconcentrator membranes was attempted using a scanning probe microscope (SPM). The samples studied were commercially available hemoconcentrator membranes PUREMA A and B (JMS Co. Ltd., Japan) having asymmetric structures. A SPM was used using the dynamic force microscopy (DFM), cyclic contact mode. The deep and tortuous pore structure on the inner surface of the hemoconcentrator membrane was observed for the first time using DFM. The pores had an elliptical shape, elongated in the longitudinal direction. When the elliptical area on the inner surface of the hemoconcentrator membrane was larger, pure water permeability was higher, showing a correlation between the elliptical area and membrane functions. The mean major pore diameters and minor pore diameters as well as the equivalent pore diameter calculated from the tortuous capillary pore model were consistent. Using DFM, the three-dimensional tortuous capillary pores at the inner surface of a hollow fiber hemoconcentrator membrane could be studied, and pore diameter and distribution could be measured by image analysis. The results were supported by the tortuous capillary pore model. In the future, we need to clearly show the further superior innovations or creative/ingenious techniques related to this study. Further the state of new findings which contribute to development of a new hemoconcentrator and other semipermeable membranes will help to increase the value of this paper. This study is one of the key studies to achieve the targeted function for the transport phenomena through semipermeable membranes including hemoconcentrator.