7. A same document matching system that forms a group of same documents among given documents, comprising: a first same document candidate group storage section in which correspondence between a same document candidate group, which is a group of documents distinguished by words or characters and including candidates of same documents, and a document belonging to said same document candidate group is stored, the first same document candidate group storage section being a non-transitory medium; a second same document candidate group storage section in which correspondence between a same document candidate group, which is a group of documents distinguished by words or characters and including candidates of same documents, and a document belonging to said same document candidate group is stored, the second same document candidate group storage being a non-transitory medium; a first same document candidate group creator that obtains a proportion of words or characters that are present in common between documents so that the documents have a similarity of at least equal to or greater than a predetermined similarity threshold, determines a selecting number of words or characters in each document based on said proportion, selects said selecting number of words or characters from said documents according to a first priority in which words or characters with a smaller number of documents where they appear have a higher priority, and stores the documents from which said words or characters have been selected into said first same document candidate group storage section, said documents being correlated with a same document candidate group distinguished by each selected word or character; a second same document candidate group creator that obtains a proportion of words or characters that are present in common between documents so that the documents have a similarity of at least equal to or greater than a predetermined similarity threshold, determines a selecting number of words or characters in each document based on said proportion, selects words or characters from said documents according to a second priority that is different from said first priority, and stores the documents from which said words or characters have been selected into said second same document candidate group storage section, said documents being correlated with a same document candidate group distinguished by each selected word or character; and a same document group creator that calculates a similarity of a pair of documents that are a pair of two different documents stored in said first same document candidate group storage section and correlated with the same document candidate group, calculates a similarity of a pair of documents that are a pair of two different documents stored in said second same document candidate group storage section and correlated with the same document candidate group, and out of pairs of documents belonging to any of said first and said second same document candidate group storage sections, registers a pair of documents that have a similarity equal to or greater than a similarity threshold as a same document group.
7. A same document matching system that forms a group of same documents among given documents, comprising: a first same document candidate group storage section in which correspondence between a same document candidate group, which is a group of documents distinguished by words or characters and including candidates of same documents, and a document belonging to said same document candidate group is stored, the first same document candidate group storage section being a non-transitory medium; a second same document candidate group storage section in which correspondence between a same document candidate group, which is a group of documents distinguished by words or characters and including candidates of same documents, and a document belonging to said same document candidate group is stored, the second same document candidate group storage being a non-transitory medium; a first same document candidate group creator that obtains a proportion of words or characters that are present in common between documents so that the documents have a similarity of at least equal to or greater than a predetermined similarity threshold, determines a selecting number of words or characters in each document based on said proportion, selects said selecting number of words or characters from said documents according to a first priority in which words or characters with a smaller number of documents where they appear have a higher priority, and stores the documents from which said words or characters have been selected into said first same document candidate group storage section, said documents being correlated with a same document candidate group distinguished by each selected word or character; a second same document candidate group creator that obtains a proportion of words or characters that are present in common between documents so that the documents have a similarity of at least equal to or greater than a predetermined similarity threshold, determines a selecting number of words or characters in each document based on said proportion, selects words or characters from said documents according to a second priority that is different from said first priority, and stores the documents from which said words or characters have been selected into said second same document candidate group storage section, said documents being correlated with a same document candidate group distinguished by each selected word or character; and a same document group creator that calculates a similarity of a pair of documents that are a pair of two different documents stored in said first same document candidate group storage section and correlated with the same document candidate group, calculates a similarity of a pair of documents that are a pair of two different documents stored in said second same document candidate group storage section and correlated with the same document candidate group, and out of pairs of documents belonging to any of said first and said second same document candidate group storage sections, registers a pair of documents that have a similarity equal to or greater than a similarity threshold as a same document group. 11. A same document matching system according to claim 7 , wherein: said same document candidate group creator and a plurality of said same document candidate group storage sections are provided for each of different priorities; and said same document group creator is configured to define a pair of documents having a similarity equal to or greater than the similarity threshold as members of a same document group, among pairs of documents belonging to an identical same document candidate group in any one of all of a plurality of the same document candidate group storage sections.