The tissue structure of the ducts in the human nipple is not completely understood as of 2019. For example, although the number of ducts in the nipple which is basic information is 15 to 20 in the textbook on anatomy, the average value in reports after 2000 is about 25, over 50 in case of large number. Although there are some reposts relate to the structure of ducts, statistical analysis with many cases has hardly been performed because such anatomic analysis is based on three-dimensional (3D) pathological observations that require great efforts. In this study, nipples of 23 cases taken from breast of women were imaged by the refractometric contrast computed tomography (CT) based on X-ray dark-field imaging which can be visualized with contrast comparable to stained two-dimensional histological section. Then, 3D visualization and the analysis of the structure of ducts in nipple are performed and compared with the previous research based on pathological method to show that the refraction-contrast CT is useful in micro anatomical research.