A document management method includes shifting a character string of characters from document data and clipping it, determining that a management Gram obtained by the clipping is one of a first Gram of low frequency and a second Gram of high frequency, storing first post data in a first post region in association with a Gram value obtained by computing the character string of first Gram, the first post data having a set of a document identification (ID) indicating the document data including the first Gram and an intra-document offset indicating a character string position thereof, and storing second post data in a second post region in association with the character string of second Gram, the second post data having a set of a document identification (ID) indicating document data including the second Gram and an intra-document offset indicating a character string position thereof.