1. A computer-readable recording medium containing a form processing program for extracting prescribed keywords from a form image scanned, the form processing program causing a computer to perform as: a layout recognizer which recognizes a layout of the form image and extracts a readout region containing character images from the form image; a character recognizer which recognizes characters from the character images of the extracted readout region through character recognition, and outputs the recognized characters as a character recognition result; a possible string extractor which extracts from the character recognition result characters which are included in strings defined as the keywords in form logical definitions, and determines as possible strings combinations of extracted characters each of which satisfies positional relationships of one of the strings, the form logical definitions defining the strings as the keywords according to logical structures which are common to forms of same type; and a linking unit which, for the keywords each represented by a plurality of elements, links the possible strings belonging to different elements of the plurality of elements according to positional relationships on the form image, and determines a combination of possible strings as the keywords.