1. An equivalence determination system comprising: a specifying unit that specifies, as rarely manually modified parts, parts of respective electronic documents in a set of electronic documents at which objects indicating a text, a figure, and an equation in electronic documents exist densely; and a determination unit that determines whether the parts specified by said specifying unit match each other between a plurality of electronic documents, and when determining that the parts match each other, specifies that the documents are a plurality of similar electronic documents.