1. A searching apparatus comprising: a memory unit which stores, for each of n-grams (where n is a natural number) extracted from plural pieces of document data subjected to searching, a transposed index representing an appearing position in the plural pieces of document data and an appearing frequency therein, the n-gram being a character string including n number of characters; an n-gram extracting unit that extracts all n-grams which are extractable from a searching character string; a smallest-frequency deriving unit which refers to the appearing frequency of the n-gram represented by the transposed index, and which derives an n-gram with a smallest appearing frequency among all of the n-grams extracted by the n-gram extracting unit; a searching n-gram selecting unit that selects, from all of the n-grams extracted by the n-gram extracting unit, a plurality of searching n-grams which form the searching character string and which include the n-gram with the smallest appearing frequency derived by the smallest-frequency deriving unit; and a document specifying unit that specifies, based on the plurality of searching n-grams selected by the searching n-gram selecting unit and the appearing position of the searching n-gram represented by the transposed index, document data including the searching character string among the plural pieces of document data.