In this study, we consider the relationship between red blood cell velocity and the movement of erythrocytes. Many velocimeters measure fluid velocity by tracking the movement of tracer particles present in the fluid. Generally, in laser Doppler velocimetry, seeding particles of suitable density and size are added to the fluid being measured, but this is not possible for in vivo measurements. In the case of blood, erythrocytes are used as the tracer particles. It is unclear, however, whether the velocity of erythrocytes reflects actual red blood cell velocity. Therefore, we compare the results of flow velocity distribution measurements of blood in the ear vessels of mouse and in water containing tracer particles in a serpentine flow channel. Results of the comparison indicate that the velocity of moving erythrocytes does not correspond exactly to the velocity of the blood. The difference is due to the changes in the fringe pattern under the skin. However, we consider that it is possible to measure red blood cell velocity in any direction using these fringe changes. Moreover, we confirm that results obtained using the proposed method match those obtained using other methods. (C) 2016 Institute of Electrical Engineers of Japan. Published by John Wiley & Sons, Inc.