10. The search method according to claim 9 , wherein of the extracted N-gram strings, the N-gram strings containing the delimiting characters each include two or more characters.
10. The search method according to claim 9 , wherein of the extracted N-gram strings, the N-gram strings containing the delimiting characters each include two or more characters. 11. The search method according to claim 10 , wherein the document data is stored in a memory area of a memory, and the memory further includes a memory area which stores an index showing the position where each of the N-gram strings contained in multiple search target character strings and containing a prescribed number of characters appears in the search target character string; wherein said designating the appearance positions comprises designating the positions where the N-gram strings appear using the index.