1. An information search apparatus comprising: a processor; a memory that stores an index database (DB); 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 the index DB; a similarity calculation unit, implemented by the processor, 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 index DB stores each word with sentence-based occurrence position information, of each document where the word occurs, that indicates each position of sentences where the word occurs, and when the input character string contains a plurality of words, the character string information search unit searches the index DB, based on each word, to obtain a document d including each word and occurrence positions of each word in the document d, and the similarity calculation unit calculates a degree of agreement score(Q,d,k) between occurrence positions of the words by score ( Q , d , k ) ⁢ ∑ q i ∈ Q ⁢ ⁢ ∑ q j ∈ ( Q - q i ) ⁢ ⁢ 1 α ⁢ ⁢ k + 1 ⁢ count ⁡ ( Pos d ⁡ ( q i ) , Pos d k ⁡ ( q j ) ) so as to calculate the degree of similarity based on the degree of agreement score (Q,d,k), wherein Q indicates a set of words obtained by dividing the character string, Pos d (q i ) indicates an occurrence position of a word q i in the document d, Pos dk (q i ) indicates a value obtained by subtracting k from an occurrence position of a word q i in the document d, k indicates a counter value, α indicates a coefficient, and Count (Pos,Pos) indicates a function for receiving two pieces of position data and returning a degree of agreement.