1. A method of speech recognition based interactive information retrieval for ascertaining and retrieving a target information of a user by determining a retrieval key entered by the user using a speech recognition processing, comprising the steps of: (a) storing retrieval key candidates that constitute a number of data that cannot be processed by the speech recognition processing in a prescribed processing time, as recognition target words in a speech recognition database, the recognition target words being divided into prioritized recognition target words that constitute a number of data that can be processed by the speech recognition processing in the prescribed processing time and that have relatively higher importance levels based on statistical information defined for the recognition target words, and non-prioritized recognition target words other than the prioritized recognition target words; (b) requesting the user by a speech dialogue with the user to enter a speech input indicating the retrieval key, and carrying out the speech recognition processing for the speech input with respect to the prioritized recognition target words to obtain a recognition result; (c) carrying out a confirmation process using a speech dialogue with the user according to the recognition result to determine the retrieval key, when the recognition result satisfies a prescribed condition for judging that the retrieval key can be determined only by a confirmation process with the user; (d) carrying out a related information query using a speech dialogue with the user to request the user to enter another speech input indicating a related information of the retrieval key, when the recognition result does not satisfy the prescribed condition; (e) carrying out the speech recognition processing for the another speech input to obtain another recognition result, and adjusting the recognition result according to the another recognition result to obtain adjusted recognition result; and (f) repeating the step (c) or the steps (d) and (e) using the adjusted recognition result in place of the recognition result, until the retrieval key is determined.