1. An information retrieval device comprising a control unit that: when a keyword for search is entered, collects texts containing that keyword from texts stored in a storage unit; extracts a noun of collected first texts; determines a noun partially matching with the keyword as a first word; extracts a second text containing that first word among the first texts; extracts a word from the second text, the word being one of a noun, a verb, and an adjective; counts the number of times the extracted word is used; determines a word whose number of times of use is placed in predefined highest ranks as a second word, the second word being a related word to the first word; and outputs the first word and the second word.