1. A method for processing an image including a character, comprising: searching, by a processor, in a set of characters one or more characters having highest similarities of shape to a character in the set of characters, hereinafter the character being referred to as a first character, the one or more searched characters forming a similar character list of the first character; searching, by the processor, in the set of characters one or more characters having highest similarities of shape to each character in the similar character list of the first character, to form a similar character list of each character in the similar character list of the first character; and selecting, by the processor, in the similar character lists one or more characters having a high mutual similarity between each other, as a character cluster.