4. An information retrieval method comprising: when a keyword for search is entered, collecting, by a computing device, texts containing the keyword from texts stored in a storage unit; extracting, by the computing device, a noun of the collected first texts; determining, by the computing device, a noun partially matching with the keyword as a first word; extracting, by the computing device, a second text containing the first word from the first texts; extracting, by the computing device, a word from the second text, the word being one of a noun, a verb, and an adjective; counting, by the computing device, a number of times a word extracted from the second text is used; determining from the second text, by the computing device, a word extracted from the second text having a 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 outputting, by the computing device, the first word and the second word, wherein a word other than the first word among nouns extracted from the first text is determined as a third word, a fourth text containing the third word is extracted from the first text, a word that is at least one of a noun, a verb, and an adjective is extracted from the fourth text, a number of times the word that is at least one of a noun, a verb, and an adjective is extracted from the fourth text is used is counted, a word having a number of times of use that is in predefined highest ranks is determined as a fourth word that is a related word to the third word, the third word and the fourth word are output, a general-purpose word list listing a plurality of words that are to be deleted from the words extracted from the second text, and with regard to the extracted word, a rank of a given word that matches with a word contained in the general-purpose word list is lowered or the given word is deleted 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.
4. An information retrieval method comprising: when a keyword for search is entered, collecting, by a computing device, texts containing the keyword from texts stored in a storage unit; extracting, by the computing device, a noun of the collected first texts; determining, by the computing device, a noun partially matching with the keyword as a first word; extracting, by the computing device, a second text containing the first word from the first texts; extracting, by the computing device, a word from the second text, the word being one of a noun, a verb, and an adjective; counting, by the computing device, a number of times a word extracted from the second text is used; determining from the second text, by the computing device, a word extracted from the second text having a 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 outputting, by the computing device, the first word and the second word, wherein a word other than the first word among nouns extracted from the first text is determined as a third word, a fourth text containing the third word is extracted from the first text, a word that is at least one of a noun, a verb, and an adjective is extracted from the fourth text, a number of times the word that is at least one of a noun, a verb, and an adjective is extracted from the fourth text is used is counted, a word having a number of times of use that is in predefined highest ranks is determined as a fourth word that is a related word to the third word, the third word and the fourth word are output, a general-purpose word list listing a plurality of words that are to be deleted from the words extracted from the second text, and with regard to the extracted word, a rank of a given word that matches with a word contained in the general-purpose word list is lowered or the given word is deleted 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. 6. The information retrieval method according to claim 4 , wherein upon receiving a general-purpose word extraction request or at a predefined time, a text stored in the storage unit is collected, a word is extracted from the collected text, a word having an appearance frequency during a first predefined period is higher than a first predetermined value and having an appearance frequency during each second predefined period shorter than the first predefined period fluctuates within a second predetermined value range is determined as a general-purpose word, and the general-purpose word list that is a list of the general-purpose word is created.