1. A method of processing an image by segmenting the image into a plurality of clusters, comprising: a cluster generation step of allocating, when a distance between a feature vector of a processing target pixel which is selected from the image sequentially and a representative feature vector of a cluster which has the nearest representative feature vector to the feature vector of the processing target pixel in a plurality of pre-defined clusters is not more than a first distance threshold, the processing target pixel to the cluster which has the nearest representative feature vector to the feature vector of the processing target pixel in the plurality of pre-defined clusters, while, when the distance between the feature vector of the processing target pixel and the representative feature vector of the cluster which has the nearest representative feature vector to the feature vector of the processing target pixel in the plurality of pre-defined clusters is more than the first distance threshold, defining a new cluster and allocating the processing target pixel to the new defined cluster; and a cluster merging step of merging, every after processes in said cluster generation step have been performed on a predetermined number of processing target pixels, a small cluster to a cluster having the nearest representative feature vector to a representative feature vector of the small cluster in the plurality of pre-defined clusters, wherein the small cluster is a cluster of which a number of allocated pixels is not more than a pixel count threshold.