7. A searching method using a searching apparatus which stores, for each of a plurality of n-grams (where n is a natural number) extracted from plural pieces of document data subjectable to searching, a transposed index representing appearing positions in the plural pieces of document data and an appearing frequency, wherein each n-gram is a character string including n number of characters, the searching method comprising: an n-gram extracting step of extracting all n-grams which are extractable from a searching character string; a smallest-frequency deriving step of referring to the appearing frequencies of the plurality of n-grams represented by the transposed indexes, and deriving an n-gram with a smallest appearing frequency among all of the n-grams extracted through the n-gram extracting step; a searching n-gram selecting step of: (a) selecting an n-gram including a first or last character of the searching character string among all of the n-grams extracted through the n-gram extracting step, (b) additionally selecting the n-gram with the smallest appearing frequency derived through the smallest-frequency deriving step, and (c) dividing the searching character string n-gram by n-gram so that one n-gram does not overlap with another n-gram with reference to a position of the n-gram with the smallest appearing frequency in the searching character string in a direction frontward or backward of that position, and additionally selecting a divided n-gram not selected through (a); and a document specifying step of specifying, based on the plurality of searching n-grams selected through the searching n-gram selecting step and based on the appearing positions of the searching n-grams represented by the transposed indexes, document data including the searching character string among the plural pieces of document data.
8. The searching method according to claim 7 , wherein using the plurality of selected searching n-grams in an order of smaller appearing frequency represented by the transposed indexes, the document data including the searching character string is specified among the plural pieces of document data based on whether or not all of the other searching n-grams are present at an appearing position of a searching n-gram and at a following appearing position.