1. An information search apparatus comprising: a character string input unit configured to obtain a character string from a client; a character string information search unit configured to obtain information that includes the character string from an index DB; a similarity calculation unit configured to calculate degree of similarity between the character string and searched information; and an output unit configured to output the searched information in descending order of the degree of similarity, wherein the character string information search unit includes a unit configured to, when the input character string contains a plurality of words, search an index DB, based on each word, that stores words and occurrence position information of the words to obtain a distance between occurrence positions of the words, and the similarity calculation unit includes a unit configured to calculate the degree of similarity based on the distance between occurrence positions of the words.