1. A non-transitory computer-readable recording medium storing therein an information searching program that causes a computer to execute a process, the process comprising: receiving one or a plurality of search keywords used for searching from a plurality of data items; specifying a first group of data items utilizing a character appearance map stored in a storage, wherein each of the first group of data items includes all characters constituting the one or a plurality of search keywords, the character appearance map indicating whether each character included in the plurality of data items is included in the plurality of data items, respectively; and searching the one or plurality of search keywords for the first group of data items, wherein the specifying specifies a second group of data items that each include a head character of the one or the plurality of search keywords, based on a head character appearance map stored in the storage, the head character appearance map indicating whether each character in the head character appearance map is included as a head character respectively in the plurality of data items, wherein the specifying also specifies a fourth group of data items from the second group of data items that include both each head character and each tail character of the one or plurality of search keywords, based on a tail character appearance map indicating whether each character in the tail character appearance map is included as a head character respectively in the plurality of data items, wherein the searching searches the one or plurality of search keywords utilizing the second group of data items, and wherein the searching also searches the one or plurality of search keywords utilizing the fourth group of data items.