1. An information retrieval apparatus that retrieves a piece of text from plural pieces of text to be searched, said information retrieval apparatus comprising: a memory; an associative dictionary storage unit that stores, using the memory, (i) two arbitrary words from among words included in the plural pieces of text to be searched and (ii) a relevance degree indicating a strength of relevance between the two arbitrary words and having a larger value when the relevance between the two arbitrary words is stronger, wherein the two arbitrary words and the relevance degree are stored in association with each other; an appearance frequency storage unit that stores (i) a word from among the words stored in said associative dictionary storage unit and (ii) an appearance frequency indicating a number of pieces of text, from among the plural pieces of text to be searched, in which the word appears, wherein the word and the appearance frequency are stored in association with each other; a search word acceptance unit configured to accept a search word; a user search intent obtaining unit configured to obtain a user search intent that indicates a search intent of a user; a user search intent threshold storage unit that stores (i) an unknown boom acquisition mode corresponding to a search intent of the user intending to retrieve a piece of text including a word that is unpredictable with respect to the search word and currently in fashion, (ii) an unknown boom acquisition mode relevance degree threshold condition that a relevance degree is equal to or below an unpredictability threshold, and (iii) an unknown boom acquisition mode appearance frequency threshold condition that an appearance frequency is equal to or above a frequency threshold, wherein the unknown boom acquisition mode, the unknown boom acquisition mode relevance degree threshold condition, and the unknown boom acquisition mode appearance frequency threshold condition are stored in association with each other; a related word obtaining unit configured to obtain, as a related word relevant to the search word, a first target word having (i) a relevance degree with respect to the search word that satisfies the unknown boom acquisition mode relevance degree threshold condition that the relevance degree is equal to or below the unpredictability threshold and (ii) an appearance frequency that satisfies the unknown boom acquisition mode appearance frequency threshold condition that the appearance frequency is equal to or above the frequency threshold, when said user search intent obtaining unit obtains the unknown boom acquisition mode, wherein the relevance degree for the first target word is stored in said associative dictionary storage unit and the appearance frequency for the first target word is stored in said appearance frequency storage unit; and a retrieval unit configured to retrieve a piece of text including the search word or the related word from the plural pieces of text to be searched.