1. A method for processing a document including an internal structure made up of a plurality of elements, comprising: a characteristic information extracting step of extracting characteristics of said document; and a document classification step of classifying each document into a plurality of classification items making up a classification model depending on the degree of interrelation between the characteristic information of the document extracted by the characteristic information extracting step and the classification-item-based characteristic information, wherein a weight is imparted to each element based on the internal structure of said document, and wherein said characteristic information extracting step includes extracting an element having the weight larger than a pre-set value.