7. A non-transitory computer-readable data storage medium storing a program to be executed by a computer, the program being configured to cause the computer to execute processes comprising: when a keyword for search is entered, collecting texts containing the keyword from texts stored in a storage unit; extracting a noun of collected first texts; determining a noun partially matching with the keyword as a first word; extracting a second text containing the first word among the first texts; extracting a word from the second text, the word being one of a noun, a verb, and an adjective; counting a number of times a word extracted from the second text is used; determining from the second text 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; outputting the first word and the second word; determining a word other than the first word among nouns extracted from the first text as a third word; extracting a fourth text containing the third word from the first text; extracting a word that is at least one of a noun, a verb, and an adjective from the fourth text; counting a number of times the word extracted from the fourth text is used; determining 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; outputting the third word and the fourth word; storing 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, lowering a rank of a word that matches with a word contained in the general-purpose word list or deleting the 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.
7. A non-transitory computer-readable data storage medium storing a program to be executed by a computer, the program being configured to cause the computer to execute processes comprising: when a keyword for search is entered, collecting texts containing the keyword from texts stored in a storage unit; extracting a noun of collected first texts; determining a noun partially matching with the keyword as a first word; extracting a second text containing the first word among the first texts; extracting a word from the second text, the word being one of a noun, a verb, and an adjective; counting a number of times a word extracted from the second text is used; determining from the second text 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; outputting the first word and the second word; determining a word other than the first word among nouns extracted from the first text as a third word; extracting a fourth text containing the third word from the first text; extracting a word that is at least one of a noun, a verb, and an adjective from the fourth text; counting a number of times the word extracted from the fourth text is used; determining 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; outputting the third word and the fourth word; storing 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, lowering a rank of a word that matches with a word contained in the general-purpose word list or deleting the 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. 9. The non-transitory computer-readable data storage medium according to claim 7 , wherein the program is further configured to cause the computer to execute processes for, upon receiving a general-purpose word extraction request or at a predefined time, collecting a text stored in the storage unit, extracting a word from the collected text, determining 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 as a general-purpose word, and creating the general-purpose word list that is a list of the general-purpose word.