6. The computer-readable storage medium as claimed in claim 5 , wherein the plurality of search methods include a first search method for searching a document based on a relation between words included in each of the character strings in the target document, and wherein by said optimizing module, the acquired character strings are optimized into first blocks each forming a sentence as the blocks, in the case where the first search method is selected by a user.
6. The computer-readable storage medium as claimed in claim 5 , wherein the plurality of search methods include a first search method for searching a document based on a relation between words included in each of the character strings in the target document, and wherein by said optimizing module, the acquired character strings are optimized into first blocks each forming a sentence as the blocks, in the case where the first search method is selected by a user. 7. The computer-readable storage medium as claimed in claim 6 , wherein the plurality of search methods include a second search method for searching a document based on concepts of words included in each of the character strings in the target document, and wherein by said optimizing module, the acquired character strings are optimized into the first blocks and further, when at least two of the successive first blocks sharing a common concept, optimizes the at least two of the successive first blocks into a second block as one of the blocks, in the case where the second search method is selected by a user.