1. An information retrieval method, comprising: storing, in a co-occurrence information database, co-occurrence information associating a word of a predetermined part of speech and an interrogative expression with each other, the word of the predetermined part of speech being extracted as a search term from a word stream including a unit expression becoming an answer candidate in each document stored in a document database and the interrogative expression being generated by combining “How many (much)” and the unit expression; performing speech recognition for a spoken question to generate text information; detecting the search term from the text information; selecting, from a plurality of interrogative expressions corresponding to the detected search term, an interrogative expression indicating a similarity equal to or greater than a threshold value with respect to a word in the text information, and correcting the text information by adding the selected interrogative expression to the text information; and searching the document database by using the corrected text information, and extracting, from each document obtained as a search result, a word matching a type of the unit expression included in the interrogative expression in the corrected text information, as a response.