1. An image processing method for inputting a document image including a clipart image to an image processing apparatus to perform vectorization, the image processing method comprising the steps of: region-segmenting the document image and selecting the clipart image; obtaining edge strength information for pixels from the clipart image; forming an edge cluster based on pixels having the edge strength information that is higher than a threshold; segmenting pixels of the clipart image other than the formed edge cluster into a plurality of clusters based on color characteristics of the pixels; determining a new edge cluster from the plurality of clusters segmented in the segmenting step by calculating an edge ratio for each of the plurality of clusters; wherein the determining step is performed by calculating an edge ratio of a cluster other than the initially formed edge cluster, which contains edge information, determining the cluster as an edge cluster based on the magnitude of the edge ratio; and fusing the new edge cluster determined in the determining step with the edge cluster formed in the edge cluster forming step; and converting each cluster after cluster fusion to vector data based on a contour line and an internal color of each cluster.