1. A computer readable recording medium in which an inverted index is recorded, where the inverted index is a data structure storing, for every indexing term in a document set consisting of a plurality of documents respectively assigned identification numbers, postings each of which is a pair of an identification number of a document in which the indexing term occurs, and a number of occurrences of the indexing term in the document, where the indexing term is defined as either a word or a plurality of contiguous letters, wherein: in the inverted index, the identification number of a document in each posting is represented by a byte sequence of variable length, the number of occurrences from 1 to (2 w −1) is represented by w bits within the byte sequence where w is a given integer of not less than 1 and not more than 8, and the number of occurrences of not less than 2 w is represented by an additional byte sequence.