To easily determine similarity of document data of PowerPoint (R) or the like with each other.

A similarity determination device pays attention to the document title, page title and page text of the document data, and determine the similarity of the document data with each other. At this point, the similarity determination device determines whether the same document title is provided, the ratio of the number of pages of the same page title and the ratio of the same page text among the document data, and determines of what kind of a similarity pattern it is the similar document data on the basis of the determined result. The determined result is recorded in similar document information and used for the retrieval processing of the similar document data.