A specifical system based on the digital image processing approach has been developed for analysing the two dimensional Flow field from pathline photographs. This system has two important problems to be solved. The first problem is to distinguish the overlapping pathlines, and the other is to determine the Flow direction. This system overcomes there problems by using one photograph which contain pathlines obtained by short exposure and its initial positions by instantaneous flash. On this photograph the initial parts of pathlines seem to be brighter and larger than the other parts of pathlines. These characteristics of the initial parts are utilized to distinguish the overlapping pathlines and to determine the Flow direction. The system is applied to two-dimensional water Flow around a circular cylinder and the velocity fields with reverse Flow are extracted automatically.