1. A similarity calculation method of calculating a similarity of two different pieces of information having a similarity equal to or greater than a predetermined similarity, comprising: obtaining, via at least one processor, a proportion of information elements constituting the two different pieces of information that are present in common between information so that the two different pieces of information has a similarity of at least equal to or greater than the predetermined similarity; determining, via the at least one processor, a selecting number of information elements which is a number of information elements making the similarity of the two different pieces of information less than the predetermined similarity based on said proportion; selecting, via at least one processor, said selecting number of information elements from said each piece of information in accordance with a first priority in which an information element with a smaller number of said information where it appears has a higher priority; selecting via at least one processor, a first group of two different pieces of information having no common selected information element between the two different pieces of information; selecting, via at least one processor, said selecting number of information elements from said each piece of information in accordance with a second priority, criteria of which are different from those of said first priority; selecting, via at least one processor, a second group of two different pieces of information having no common selected information element between the two different pieces of information; and performing similarity calculation on two different pieces of information except those included in any of said first group and said second group.
3. A similarity calculation method according to claim 1 , wherein: said second priority is a priority in which an information element with a smaller number of said information where it appears has a higher priority, and an information element, which was rarely selected as the information element in selecting said first pair of information, has a higher priority.