1. A topic map based indexing apparatus, comprising: a question/answer (Q/A) pre-processing block to, by a computer, normalize community Q/A lists for plural different community sites on web, respectively, correct grammatical errors, and remove insignificant symbols, to thereby provide pre-processed community Q/A lists for the plural different community sites, respectively; a Q/A analysis block to, by a computer, analyze the pre-processed community Q/A lists to acquire Q/A analysis information; and a Q/A storage block to, by a computer, store, in a community Q/A topic map, index information containing the pre-processed community Q/A lists and the Q/A analysis information, wherein the indexing information is obtained by removing redundant answers depending on the Q/A analysis information, removing insignificant answers based on a degree of reliability, ranking answer lists, extracting a highest ranking answer as a best answer, and determining a topic, wherein the Q/A pre-processing block includes: a one unit recognizing unit to normalize, in a single form, allomorph words or words recognizable as one unit in the community Q/A lists by referring to a unit dictionary database; and an error pre-processing unit to correct the grammatical errors and remove the insignificant symbols from the community Q/A lists by referring to a pre-processing rule database.