A data search method for performing a data search that searches data stored in a data storage for a search string input from an input unit, the method comprising: receiving, for use in the data search, a search string from the input unit; then (a) acquiring a number of data in the data storage that matches a keyword (i) corresponding to a substring of the search string from the beginning of the search string to a next space in the search string, if a space is present, or (ii) corresponding to the whole search string, if a space is not present; then (b) determining whether or not the acquired number of data is equal to or greater than a predetermined number; then (c) determining (S3, S7) whether the search string includes a space; then (d) searching the data storage for data by performing an AND search that finds matches for all of keywords corresponding to substrings that are divided by all spaces in the search string, when the acquired number of data is smaller than the predetermined number and it is determined in step (c) that the search string includes a space, or (e) replacing the search string by a new search string in which a first space is excluded, and repeating steps (a) to (e), when the acquired number of data is equal to or greater than the predetermined number and it is determined in step (c) that the search string includes a space; or (f) performing a single keyword search in the data storage for the search string, when it is determined in step (c) that the search string does not include a space. receiving, for use in the data search, a search string from the input unit; then (a) acquiring a number of data in the data storage that matches a keyword (i) corresponding to a substring of the search string from the beginning of the search string to a next space in the search string, if a space is present, or (ii) corresponding to the whole search string, if a space is not present; then (b) determining whether or not the acquired number of data is equal to or greater than a predetermined number; then (c) determining (S3, S7) whether the search string includes a space; then (d) searching the data storage for data by performing an AND search that finds matches for all of keywords corresponding to substrings that are divided by all spaces in the search string, when the acquired number of data is smaller than the predetermined number and it is determined in step (c) that the search string includes a space, or (e) replacing the search string by a new search string in which a first space is excluded, and repeating steps (a) to (e), when the acquired number of data is equal to or greater than the predetermined number and it is determined in step (c) that the search string includes a space; or (f) performing a single keyword search in the data storage for the search string, when it is determined in step (c) that the search string does not include a space. (a) acquiring a number of data in the data storage that matches a keyword (i) corresponding to a substring of the search string from the beginning of the search string to a next space in the search string, if a space is present, or (ii) corresponding to the whole search string, if a space is not present; then (b) determining whether or not the acquired number of data is equal to or greater than a predetermined number; then (c) determining (S3, S7) whether the search string includes a space; then (d) searching the data storage for data by performing an AND search that finds matches for all of keywords corresponding to substrings that are divided by all spaces in the search string, when the acquired number of data is smaller than the predetermined number and it is determined in step (c) that the search string includes a space, or (e) replacing the search string by a new search string in which a first space is excluded, and repeating steps (a) to (e), when the acquired number of data is equal to or greater than the predetermined number and it is determined in step (c) that the search string includes a space; or (f) performing a single keyword search in the data storage for the search string, when it is determined in step (c) that the search string does not include a space.