1. An image processing method comprising: using an image processing apparatus to perform the following steps: a segmentation step of segmenting a document image into a plurality of areas; a selection step of selecting an illustration area from among the plurality of areas; a clustering step of clustering the illustration area selected, and generating a plurality of clusters; a determination step of determining whether a small area included in the generated clusters is noise; an elimination step of eliminating a small area determined as being noise by combining the noise small area with a cluster determined as being similar among clusters adjacent to the small area; and a conversion step of converting the cluster from which the noise small areas have been eliminated in said elimination step into vector data.