In order to clarify the relation of the erythrocyte sedimentation rate (ESR) with the degree of the aggregation of erythrocyte, we have developed a microscopic image analyzing system and applied it to anticoagulated swine blood. Erythrocyte-poor domains are found to be made up in the process of sedimentation. The fraction of the domain φ initially increased, reached a maximum, and then decreased gradually. The surface variation Y of the interface between the supernatant plasma and the erythrocyte suspension varied similarly to φ. Also, ESR was the maximum just when φ and Y were the maximum. These results suggest that as time passes, erythrocyte-poor domains are produced due to the aggregation of erythrocytes and plasma is carried upward effectively through the domains, resulting in ESR as well as the surface variation having their maxima.