1. A speech processing device comprising: an utterance error occurrence determination information storage unit configured to store utterance error occurrence determination information in which error patterns are associated with conditions of a word causing an utterance error; a related word information storage unit configured to store related word information including words, which are likely to cause a speech error, for each word that causes the utterance error, the speech error being an error in which, after a wrong word is completely or partially uttered, a correct word is uttered, or the speech error being an error in which the wrong word is uttered without any correction; a character string analyzing unit configured to linguistically analyze a character string and divides the character string into word strings; an utterance error occurrence determining unit configured to compare each of the divided words with the conditions, give the error pattern to a word corresponding to the conditions, and determine that a word which does not correspond to the conditions does not cause the utterance error; and a phoneme string generating unit configured to generate a phoneme string of the utterance error corresponding to the error pattern in the word having the error pattern given thereto and generate a general phoneme string in the word that is determined not to cause the utterance error, thereby generating a phoneme string of the word strings, wherein one of the error patterns associated with one of the conditions is the speech error, when there is a certain word having the speech error as the error pattern, the utterance error occurrence determining unit further gives the certain word an incorrectly spoken word selected from the related word information, and the phoneme string generating unit generates, as the phoneme string of the utterance error corresponding to the error pattern of the certain word, a phoneme string including at least a part of the incorrectly spoken word and, subsequent to at least the part of the incorrectly spoken word, the certain word.