1. An information retrieval device comprising: a computer; a control unit implemented at least by the computer and that: when a keyword for search is entered, collects first texts containing the keyword from texts stored in a storage unit implemented at least by the computer; extracts a noun of the collected first texts; determines a noun partially matching with the keyword as a first word; extracts a second text containing the first word, from the first texts; extracts a word from the second text, the word being one of a noun, a verb, and an adjective; counts a number of times the extracted word is used; determines, from the second text, a word having a number of times of use in predefined highest ranks as a second word, the second word being a related word to the first word; outputs the first word and the second word; and a memory unit implemented at least by the computer and that stores a general-purpose word list listing a plurality of words that are to be deleted from the words extracted from the second text, wherein the control unit lowers, with regard to the extracted word, a rank of a given word that matches with a word contained in the general-purpose word list or deletes the given word from the words extracted from the second text by referring to the general-purpose word list, the rank of the given word being lowered with respect to the number of times the given word extracted from the second text is used as counted, and the control unit determines a word other than the first word among nouns extracted from the first text as a third word, extracts a fourth text containing the third word from the first text, extracts a word that is at least one of a noun, a verb, and an adjective from the fourth text, counts the number of times the extracted word from the fourth text is used, determines a word having a number of times of use that is in predefined highest ranks as a fourth word that is a related word to the third word, and outputs the third word and the fourth word.