2. An equivalence determination system comprising: a processor; an object extracting unit, executed on the processor, that extracts, from respective electronic documents in a set of electronic documents, at least one object which forms the electronic document and includes at least one of a text, a figure, and an equation; a specifying unit that specifies predetermined number of objects in the respective electronic documents based on density calculated by referring to the extracted objects; a judging unit that judges that plural electronic documents are similar based on the specified objects; a hash value calculation unit that calculates hash values of the objects specified by said specifying unit; and a feature word extraction unit that extracts a feature character string from the objects specified by said specifying unit, wherein said hash value calculation unit calculates a hash value based on the feature character string extracted by said feature word extraction unit, wherein said judging unit determines, by using the hash values calculated by said hash value calculation unit, whether the objects specified by said specifying unit match each other, wherein said specifying unit calculates, based on a density of the at least one object extracted by the object extracting unit, an improbability of modifying the at least one object, and specifies objects based on a calculation result, said feature word extraction unit extracts at least one feature word as the feature character string from the specified object by said specifying unit, said hash value calculation unit calculates a hash value of a character string obtained by concatenating feature words extracted by said feature word extraction unit, and registers identification information of corresponding electronic documents in a hash table, and said judging unit judges, based on a match between hash values, that the corresponding electronic documents are equivalent.
7. An equivalence determination system according to claim 2 , wherein said specifying unit calculates the density of a given object, from among the objects, based on an area of the given object, and an amount of character string and decoration information contained in the given object.