An indexing method is based on a tree structure of a flash memory, which includes a plurality of pages. The indexing method stores an entry in the leaf node and an entry in an index node designating the leaf node, in the same page, and changes the maximum number of entries that are stored in the leaf node of the page and the maximum number of entries that are stored in the index node of the page on the basis of the number of entries in the leaf node and the number of entries in the index node, respectively.