AbstractThis paper describes a fast labeling algorithm which separates regions lying all over an image based on a pipeline method. In the previous labeling algorithm of raster‐scan type, a bottleneck for fast processing was the label classification process to classify temporarily assigned labels based on the connectivity. To cope with this problem, consideration was given to how to reduce the number of connectivities input to the labeling process, and how to make the labeling process efficient.For the problem of the number of connectivities, a method was developed for reducing the number of connectivities due to reshaping of a region, and a fast method for reducing the number of duplicate connectivities by introduction of a dividing process. Also, for the problem of label classification, a label classification method was developed using direct search for connectivities. Moreover, the relationship between the number of provisional labels in the labeling process and the number of connectivities was investigated. As a result, it was shown that the label classification process can be implemented in time proportional to the number of provisional labels and the effectiveness of the algorithm presented here was verified by simulation.